test(document): read the editor through a module-set helper before it is split

static/js/document.js is 17,579 lines and is about to be decomposed behind a
re-export wrapper. 51 test files read it off disk and grep it as text, and 28
of those slice it with `src.split("function a", 1)[1].split("function b", 1)[0]`
-- "the region between a and b", which only means what the test intends while a
and b are neighbours in one file. Several also hard-code the file's two-space
indentation, which no extracted module reproduces. Left alone, the first
extraction makes those assertions cover the wrong region, and an `x in region`
check passes while covering more than it was written for.

tests/helpers/document_source is the one place that names the file now:

  - document_source() is the entry plus everything under static/js/document/,
    so a membership assertion keeps finding its subject wherever it lands;
  - function_body()/declaration() locate a construct by name in whichever
    module defines it and end at its real closing brace, so neither moving it
    nor moving its neighbour changes the region.

The rewrite only collapses a slice when the old terminator sat at the
construct's end. 28 slices deliberately span a whole family of functions --
everything from _docxHexColor to exportAsDocx -- and collapsing one to its
first member drops what the assertions look for, so those stay as they are and
are listed in KNOWN_ADJACENCY_SLICES, to be converted as each family becomes a
module. That list may only shrink.

Two guards come with it:

  - test_document_source_test_hygiene fails on a direct read of the entry file
    and on any new adjacency slice;
  - test_frontend_module_graph resolves every relative import under static/
    (718 of them, none broken today) and requires the document module set to
    stay in the sw.js precache, since the worker fetches the URLs it lists and
    not what they import.

test_document_module_api pins the 38 default-export keys and 29 named exports
by loading the module in a browser and reading what it actually exports, rather
than grepping for the literal object -- after extraction that object may be
assembled from imports, and a source-shape check would pass while the export
was broken.

No JavaScript moves here. static/ is untouched.

tests/test_document_active_restore.py

@@ -1,11 +1,10 @@
 """Regression guards for restoring a chat's exact active document."""
 
 from pathlib import Path
+from tests.helpers.document_source import document_source, function_body
 
 
-DOC_JS = (
-    Path(__file__).resolve().parents[1] / "static/js/document.js"
-).read_text(encoding="utf-8")
+DOC_JS = document_source()
 
 
 def test_active_document_is_persisted_per_session():
@@ -32,9 +31,7 @@ def test_closing_active_document_clears_stale_restore_pointer():
 
 
 def test_explicit_document_open_clears_minimized_dock_state():
-    ensure_mounted = DOC_JS.split("function _ensureDocPaneMounted()", 1)[1].split(
-        "export async function loadDocument", 1
-    )[0]
+    ensure_mounted = function_body("_ensureDocPaneMounted")
 
     assert "Modals.isMinimized('doc-panel')" in ensure_mounted
     assert "Modals.unregister('doc-panel');" in ensure_mounted
@@ -42,9 +39,7 @@ def test_explicit_document_open_clears_minimized_dock_state():
 
 
 def test_library_open_intent_is_persisted_before_delayed_session_restore():
-    body = DOC_JS.split("export function prepareDocumentOpen(sessionId)", 1)[1].split(
-        "/** Switch chat", 1
-    )[0]
+    body = function_body("prepareDocumentOpen")
 
     assert "_markDocVisibleState(sessionId, 'open');" in body
     assert "Modals.isMinimized('doc-panel')" in body