test(media): stop asserting an optional ffmpeg webp encoder

test_inspect_media_exports_final_decodable_frame_at_exact_duration exported to
/workspace/final.webp and asserted exit_code == 0. WebP encoding is an ffmpeg
build option, not something this project requires - Homebrew's macOS ffmpeg is
built without it:

  ffmpeg -encoders | grep -ic webp   ->  0

so the tool returns "ffmpeg still extraction failed: ... Encoder not found"
and the test fails on the build rather than on the code under test.

The test's subject is the final frame being decodable at the exact duration,
which has nothing to do with the container. It now writes a PNG, and the two
things the WebP path was implicitly covering are split out and each guarded on
what is actually present:

- test_inspect_media_exports_a_webp_still - skipped unless ffmpeg reports a
  webp encoder, and now asserts the file really is WebP rather than merely
  non-empty.
- test_inspect_media_reports_a_missing_encoder_instead_of_crashing - runs only
  where the encoder is absent, pinning the behaviour that surfaced this: the
  tool reports ffmpeg's failure as a tool error and writes no partial file.

The product code has the same assumption and I left it alone. inspect_media
accepts any suffix in _IMAGE_SUFFIXES and hands the path to ffmpeg, so a .webp
request on a build without libwebp fails with ffmpeg's own message. That is a
poor message, not a crash or a corrupt file, and pre-validating the encoder
list is a separate change.

  pytest tests/test_inspect_media_tool.py
  68 passed, 2 skipped in 22.92s      (1 failed, 66 passed, 1 skipped before)

Not verified: the WebP success path. This machine has no webp encoder, so
test_inspect_media_exports_a_webp_still skips here and has only been checked
for collection, not for a passing run.

tests/test_inspect_media_tool.py

@@ -1,5 +1,6 @@
 import asyncio
 import base64
+import functools
 import io
 import json
 from pathlib import Path
@@ -20,6 +21,28 @@ from src.tool_execution import _active_workspace
 from src.tool_schemas import FUNCTION_TOOL_SCHEMAS
 
 
+@functools.lru_cache(maxsize=None)
+def _ffmpeg_has_encoder(name: str) -> bool:
+    """Whether the ffmpeg on PATH was built with the named encoder.
+
+    Codec support is a build option, not something the project requires. The
+    Homebrew ffmpeg on macOS ships without libwebp, for instance, so a test
+    that asserts a successful `.webp` export there fails on the build rather
+    than on the tool.
+    """
+    if not shutil.which("ffmpeg"):
+        return False
+    listed = subprocess.run(
+        ["ffmpeg", "-hide_banner", "-loglevel", "error", "-encoders"],
+        check=False, capture_output=True, text=True,
+    )
+    return any(
+        line.split()[1:2] == [name]
+        for line in listed.stdout.splitlines()
+        if line.strip()
+    )
+
+
 def test_media_timestamp_parser_accepts_units_and_four_field_timecodes():
     assert _parse_seconds("0m", default=-1) == 0
     assert _parse_seconds("30m", default=-1) == 1800
@@ -491,13 +514,16 @@ def test_inspect_media_exports_final_decodable_frame_at_exact_duration(tmp_path:
         result = asyncio.run(InspectMediaTool().execute(json.dumps({
             "path": "/workspace/video.mp4",
             "timestamp": "end",
-            "output_path": "/workspace/final.webp",
+            # PNG, not WebP: this asserts that the *final* frame is decodable at
+            # the exact duration, so it must not also depend on an optional
+            # ffmpeg encoder. WebP export is covered separately below.
+            "output_path": "/workspace/final.png",
         }), {}))
     finally:
         _active_workspace.reset(token)
 
     assert result["exit_code"] == 0, result
-    assert (tmp_path / "final.webp").stat().st_size > 0
+    assert (tmp_path / "final.png").stat().st_size > 0
 
     token = _active_workspace.set(str(tmp_path))
     try:
@@ -514,6 +540,67 @@ def test_inspect_media_exports_final_decodable_frame_at_exact_duration(tmp_path:
     assert Image.open(io.BytesIO(base64.b64decode(high_detail["images"][0]["data"]))).size == (768, 432)
 
 
+@pytest.mark.skipif(not shutil.which("ffmpeg") or not shutil.which("ffprobe"), reason="ffmpeg required")
+@pytest.mark.skipif(not _ffmpeg_has_encoder("webp"), reason="ffmpeg built without a webp encoder")
+def test_inspect_media_exports_a_webp_still(tmp_path: Path):
+    """A `.webp` output_path is passed straight through to ffmpeg.
+
+    Guarded on the encoder rather than asserted unconditionally: WebP is a
+    build option (Homebrew's macOS ffmpeg omits it) and the project does not
+    require it. When the encoder is missing the tool reports ffmpeg's failure
+    with `exit_code` 1, which is covered by
+    `test_inspect_media_reports_a_missing_encoder_instead_of_crashing`.
+    """
+    video = tmp_path / "video.mp4"
+    subprocess.run([
+        "ffmpeg", "-hide_banner", "-loglevel", "error", "-f", "lavfi",
+        "-i", "testsrc2=size=320x180:rate=4:duration=2", "-pix_fmt", "yuv420p",
+        "-y", str(video),
+    ], check=True)
+    token = _active_workspace.set(str(tmp_path))
+    try:
+        result = asyncio.run(InspectMediaTool().execute(json.dumps({
+            "path": "/workspace/video.mp4",
+            "timestamp": "end",
+            "output_path": "/workspace/final.webp",
+        }), {}))
+    finally:
+        _active_workspace.reset(token)
+
+    assert result["exit_code"] == 0, result
+    assert Image.open(tmp_path / "final.webp").format == "WEBP"
+
+
+@pytest.mark.skipif(not shutil.which("ffmpeg") or not shutil.which("ffprobe"), reason="ffmpeg required")
+@pytest.mark.skipif(_ffmpeg_has_encoder("webp"), reason="needs an ffmpeg built without webp")
+def test_inspect_media_reports_a_missing_encoder_instead_of_crashing(tmp_path: Path):
+    """An export in a format this ffmpeg cannot encode fails as a tool error.
+
+    The tool does not probe the encoder list, so the only contract it can keep
+    is to surface ffmpeg's own failure rather than raise or write a truncated
+    file. Asserted only on builds that actually lack the encoder.
+    """
+    video = tmp_path / "video.mp4"
+    subprocess.run([
+        "ffmpeg", "-hide_banner", "-loglevel", "error", "-f", "lavfi",
+        "-i", "testsrc2=size=320x180:rate=4:duration=2", "-pix_fmt", "yuv420p",
+        "-y", str(video),
+    ], check=True)
+    token = _active_workspace.set(str(tmp_path))
+    try:
+        result = asyncio.run(InspectMediaTool().execute(json.dumps({
+            "path": "/workspace/video.mp4",
+            "timestamp": "end",
+            "output_path": "/workspace/final.webp",
+        }), {}))
+    finally:
+        _active_workspace.reset(token)
+
+    assert result["exit_code"] == 1
+    assert "ffmpeg still extraction failed" in result["error"]
+    assert not (tmp_path / "final.webp").exists()
+
+
 @pytest.mark.skipif(not shutil.which("ffmpeg") or not shutil.which("ffprobe"), reason="ffmpeg required")
 def test_inspect_media_rejects_ambiguous_multi_frame_single_image_export(tmp_path: Path):
     video = tmp_path / "video.mp4"