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.

agent/lsp/manager.py

@@ -40,7 +40,7 @@ import os
 import threading
 import time
 from concurrent.futures import Future as ConcurrentFuture
-from typing import Any, Dict, List, Optional, Tuple
+from typing import Any, Callable, Dict, List, Optional, Tuple
 
 from agent.lsp import eventlog
 from agent.lsp.client import (
@@ -305,6 +305,7 @@ class LSPService:
         *,
         delta: bool = True,
         timeout: Optional[float] = None,
+        line_shift: Optional[Callable[[int], Optional[int]]] = None,
     ) -> List[Dict[str, Any]]:
         """Synchronously open ``file_path`` in the right server, wait for
         diagnostics, return them.
@@ -314,6 +315,18 @@ class LSPService:
         Diagnostics present in the baseline are removed so the caller
         only sees errors introduced by the current edit.
 
+        When ``line_shift`` is provided, baseline diagnostics are
+        remapped through it before the set-difference.  This handles
+        the case where the edit deleted or inserted lines, causing
+        pre-existing diagnostics below the edit point to surface at
+        different line numbers in the post-edit snapshot — without
+        the shift, they'd all look "introduced by this edit".  Pass
+        a callable built by
+        :func:`agent.lsp.range_shift.build_line_shift` (pre_text,
+        post_text).  Omit when pre/post content isn't available;
+        the unshifted comparison still catches diagnostics that
+        didn't move.
+
         Returns an empty list when LSP is disabled, when no workspace
         can be detected, when no server matches, or when the server
         can't be spawned.  Never raises.
@@ -344,6 +357,14 @@ class LSPService:
         if delta:
             baseline = self._delta_baseline.get(abs_path) or []
             if baseline:
+                if line_shift is not None:
+                    # Remap baseline diagnostics into post-edit
+                    # coordinates so shifted-but-otherwise-identical
+                    # entries hash equal under _diag_key.  Entries
+                    # that mapped into a deleted region drop out
+                    # silently — they no longer apply.
+                    from agent.lsp.range_shift import shift_baseline
+                    baseline = shift_baseline(baseline, line_shift)
                 seen = {_diag_key(d) for d in baseline}
                 diags = [d for d in diags if _diag_key(d) not in seen]
             # Roll baseline forward — next call returns deltas relative
@@ -585,8 +606,19 @@ class LSPService:
 
 
 def _diag_key(d: Dict[str, Any]) -> str:
-    """Content equality key used for delta filtering.  Mirrors
-    :func:`agent.lsp.client._diagnostic_key`."""
+    """Content equality key used for cross-edit delta filtering.
+
+    Includes the diagnostic's position range — when used together
+    with :func:`agent.lsp.range_shift.shift_baseline`, the baseline
+    is line-shifted into post-edit coordinates BEFORE this key is
+    computed, so identical-but-shifted diagnostics hash equal.  Two
+    genuinely distinct diagnostics at different lines (e.g. the same
+    error class introduced at a second site) hash differently and
+    are surfaced as new.
+
+    Mirrors :func:`agent.lsp.client._diagnostic_key`; intentionally
+    identical so the two layers agree on diagnostic identity.
+    """
     rng = d.get("range") or {}
     start = rng.get("start") or {}
     end = rng.get("end") or {}