fix(lsp): shift baseline diagnostics into post-edit coordinates (#25978)

Pre-existing diagnostics below an edit point used to surface as 'LSP
diagnostics introduced by this edit' whenever the edit deleted or
inserted lines.  The delta-filter key included the diagnostic's
range, so the same logical error reported at a different line in
the post-edit snapshot looked like a brand new diagnostic.

Concrete case: deleting 14 lines in cli.py caused Pyright errors at
lines 9873, 10590, 12413, 13004 (unrelated to the edit) to be
reported as introduced by it.

Fix: build a piecewise-linear line-shift map (via difflib's
SequenceMatcher) from pre and post content, and remap baseline
diagnostics into post-edit coordinates before the set-difference.
Diagnostics in deleted regions drop out cleanly; diagnostics below
the edit shift by the right amount; diagnostics above are untouched.
The strict (range-aware) equality key stays — so a genuinely new
instance of an identical error class at a different line still
surfaces as new.

Pieces:
- agent/lsp/range_shift.py — build_line_shift, shift_diagnostic_range,
  shift_baseline.  Pure functions, no LSP state.
- agent/lsp/manager.py — LSPService.get_diagnostics_sync gains an
  optional line_shift kwarg; baseline is shift_baseline'd before
  computing the seen-set.  _diag_key keeps the strict range key.
- tools/file_operations.py — write_file captures pre_content for any
  LSP-handled extension (not just LINTERS_INPROC) and passes pre/post
  to _maybe_lsp_diagnostics, which builds the shift map.
- New _lsp_handles_extension helper guards the pre_content read.

Trade-offs preserved:
- Genuinely new same-class errors at different lines still surface
  (content-only key would have swallowed them).
- Pre-existing errors at unshifted positions still get filtered
  (covered by the strict-key path with no shift).
- Best-effort: when pre_content can't be captured (file didn't
  exist, permissions), the unshifted comparison still catches
  most pre-existing errors; the edge case it misses is a new file
  with a non-empty baseline, which is structurally impossible.

tests/agent/lsp/test_service.py

@@ -130,6 +130,35 @@ def test_service_e2e_delta_filter(mock_pyright):
         svc.shutdown()
 
 
+def test_service_e2e_delta_filter_with_line_shift(mock_pyright):
+    """End-to-end: an edit that shifts the diagnostic's line still
+    filters correctly when ``line_shift`` is supplied.
+
+    The mock LSP server emits a fixed error at line 0; for this test
+    we don't need to actually shift the server's output — we just
+    need to prove that supplying a line_shift through the API works
+    and doesn't break the existing delta path.  The unit tests in
+    test_delta_key.py cover the shift semantics in detail.
+    """
+    repo = mock_pyright
+    f = repo / "x.py"
+    f.write_text("print('hi')\n")
+
+    svc = LSPService(
+        enabled=True,
+        wait_mode="document",
+        wait_timeout=3.0,
+        install_strategy="manual",
+    )
+    try:
+        svc.snapshot_baseline(str(f))
+        # Identity shift — should behave exactly like no shift.
+        new_diags = svc.get_diagnostics_sync(str(f), line_shift=lambda L: L)
+        assert new_diags == []
+    finally:
+        svc.shutdown()
+
+
 def test_service_status_includes_clients(mock_pyright):
     repo = mock_pyright
     f = repo / "x.py"