fix(agent): steer GPT/Codex family to V4A for single-file edits too (#44411)

The coding-posture brief told GPT/Codex models to use patch mode='patch'
(V4A) for structured/multi-file changes but mode='replace' "for a single
small swap". That second nudge points those models at a format their
first-party harness never taught them.

Verified against openai/codex (current main): apply_patch is the ONLY file
editor in codex-rs — zero occurrences of str_replace/old_string anywhere in
the repo; the grammar (core/src/tools/handlers/apply_patch.lark) is exactly
the V4A dialect our patch_parser implements; the shipped model prompts
(gpt_5_codex, gpt-5.2-codex, gpt-5.1-codex-max + instruction templates)
explicitly say to use apply_patch "for single file edits"; and the tool is
gated per model via ModelInfo.apply_patch_tool_type, i.e. OpenAI ships
V4A-for-everything as model metadata.

The GPT-family line now steers to mode='patch' for all edits, single-file
included. The replace-family line (Claude + open-weight) is unchanged —
Claude Code's FileEdit is old_string/new_string/replace_all exact string
replacement (confirmed from Anthropic's shipped sdk-tools.d.ts, the only
file editor in its tool union), matching our mode='replace'.

tests/agent/test_coding_context.py

@@ -312,6 +312,10 @@ class TestEditFormatSteering:
         assert "mode='patch'" in brief
         assert "V4A" in brief
         assert "write_file" in brief  # new files authored, not patched
+        # Codex-family harnesses ship apply_patch (V4A) as the ONLY editor and
+        # instruct it even for single-file edits — never nudge replace mode.
+        assert "single-file" in brief
+        assert "mode='replace'" not in brief
 
     def test_anthropic_family_gets_replace_nudge(self, tmp_path):
         _git_init(tmp_path)