feat(backend): adapt every completion to the detected backend (workspace T-137)

One choke point in the sanitized client: the flavor is probed once
(the boilerroom wrapper names itself on /health, a bare llama-server
serves /props, Ollama answers neither) and every completion adapts.
The agents' tool_choice "required" survives only on Ollama — advisory
there, enforced by llama-server, an unbreakable tool loop through the
wrapper. Through the wrapper every completion carries webber's session
identity: session webber, eviction_order 20 in the decided ranking,
configurable via settings. The wrapper's balancing and compaction
signals are read from the response body's extra fields — an httpx
event-hook variant was tried and never fires under the openai SDK.

The enabling fix: the sanitized client was never in the request path.
The provider assigned self._openai_client, an attribute nobody reads —
OllamaProvider.client serves self._client — so every completion has
bypassed the null-content sanitizer since the class was introduced.
Exposed when the wrapper 503'd a session-less request the choke point
should have named; the client now goes through the constructor's
official openai_client parameter, and a wiring test pins
provider.client to the sanitized type. The same bug exists in tatlock
(its T-6, filed).

Verified against the live wrapper from the dev server: flavor
boilerroom detected, a tool-using explore run answered in 4.8 s with
no tool loop, webber resident at rank 20, and the wrapper parked
librarian and tatlock-experts to seat it — the ranking doing exactly
its job. Six new tests (227 green), five mutation-checked: the rank
default, the strip condition, the session-add condition, the
no-cache-on-failure rule, and the client wiring.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

webber-api/tests/test_provider_backend.py

@@ -0,0 +1,117 @@
+"""
+Backend adaptation at the provider choke point (workspace T-137).
+
+The flavor globals are set directly so the tests are deterministic
+regardless of which backend is reachable; the wire facts the probe
+relies on are the wrapper's contract, pinned in its own repo.
+"""
+import pytest
+
+from src.ollama import provider
+from src.shared.config import get_settings
+
+
+@pytest.fixture(autouse=True)
+def reset_flavor(monkeypatch):
+    """Each test states its flavor; nothing leaks between them."""
+    monkeypatch.setattr(provider, "_local_flavor", None)
+
+
+@pytest.mark.anyio
+async def test_ollama_keeps_the_advisory_nudge(monkeypatch):
+    monkeypatch.setattr(provider, "_local_flavor", "ollama")
+    kwargs = await provider._adapt_to_backend(
+        {"extra_body": {"tool_choice": "required"}}
+    )
+    assert kwargs["extra_body"] == {"tool_choice": "required"}
+
+
+@pytest.mark.anyio
+async def test_wrapper_strips_the_nudge_and_names_the_session(monkeypatch):
+    # tool_choice "required" is enforced by llama-server — through the
+    # wrapper it is the unbreakable tool loop, so it must not pass.
+    monkeypatch.setattr(provider, "_local_flavor", "boilerroom")
+    kwargs = await provider._adapt_to_backend(
+        {"extra_body": {"tool_choice": "required"}}
+    )
+    extra = kwargs["extra_body"]
+    assert "tool_choice" not in extra
+    assert extra["session"] == "webber"
+    assert extra["eviction_order"] == 20
+
+
+@pytest.mark.anyio
+async def test_bare_llama_server_strips_without_naming(monkeypatch):
+    monkeypatch.setattr(provider, "_local_flavor", "llama-server")
+    kwargs = await provider._adapt_to_backend(
+        {"extra_body": {"tool_choice": "required"}}
+    )
+    assert "extra_body" not in kwargs
+
+
+@pytest.mark.anyio
+async def test_rank_and_name_come_from_settings(monkeypatch):
+    monkeypatch.setattr(provider, "_local_flavor", "boilerroom")
+    settings = get_settings()
+    monkeypatch.setattr(settings, "backend_session_name", "webber-test")
+    monkeypatch.setattr(settings, "backend_session_rank", 7)
+    kwargs = await provider._adapt_to_backend({})
+    assert kwargs["extra_body"] == {"session": "webber-test", "eviction_order": 7}
+
+
+@pytest.mark.anyio
+async def test_other_extension_fields_survive_adaptation(monkeypatch):
+    monkeypatch.setattr(provider, "_local_flavor", "boilerroom")
+    kwargs = await provider._adapt_to_backend({"extra_body": {"marker": 1}})
+    assert kwargs["extra_body"]["marker"] == 1
+    assert kwargs["extra_body"]["session"] == "webber"
+
+
+def test_body_signals_are_logged(monkeypatch):
+    # The httpx event-hook variant never fired under the SDK; the body
+    # read must demonstrably log, or the signal passes silently again.
+    calls = []
+
+    class StubLogger:
+        def info(self, msg):
+            calls.append(("info", msg))
+
+        def warning(self, msg):
+            calls.append(("warning", msg))
+
+    monkeypatch.setattr(provider, "logger", StubLogger())
+
+    class Busy:
+        model_extra = {"balancing": [{"parked": "librarian"}], "compaction_due": True}
+
+    provider._log_wrapper_signals(Busy())
+    assert any(kind == "info" and "parked" in msg for kind, msg in calls)
+    assert any(kind == "warning" for kind, msg in calls)
+
+    calls.clear()
+
+    class Quiet:
+        model_extra = {"balancing": [], "compaction_due": False}
+
+    provider._log_wrapper_signals(Quiet())
+    assert calls == []
+
+
+def test_provider_serves_the_sanitized_client():
+    # The whole mechanism rides on `.client` returning OUR instance:
+    # assigning a lookalike attribute after super().__init__ built its
+    # own client is how the sanitizer silently died until 2026-09-13.
+    p = provider.WebberOllamaProvider(base_url="http://127.0.0.1:9/v1")
+    assert isinstance(p.client, provider._SanitizedAsyncOpenAI)
+
+
+@pytest.mark.anyio
+async def test_probe_failure_answers_ollama_without_caching(monkeypatch):
+    # A backend that is down at first call must be re-probed later —
+    # caching "ollama" forever would nudge llama-server into the tool
+    # loop the moment the wrapper came back.
+    settings = get_settings()
+    monkeypatch.setattr(settings, "ollama_url", "http://127.0.0.1:9")
+    flavor = await provider._detect_flavor()
+    assert flavor == "ollama"
+    assert provider._local_flavor is None