fix(tests): pin workspace snapshot in plugin-prompt-sections byte-stability test

test_real_aiagent_builds_section_once_and_keeps_it_out_of_static_prefix
builds the system prompt twice and asserts byte equality, but the prompt
embeds build_coding_workspace_block() — live git status/log output. A git
call failing between the two builds (xdist contention in CI) makes the
Branch/Recent-commits lines differ and fails the test on unrelated PRs
(first seen on #89027's run: diff showed only '- Branch: (detached HEAD)'
and recent-commit lines).

Mechanism reproduced locally: with coding posture on and cwd inside the
checkout, failing git calls on the second build only => first != rebuilt;
with the snapshot pinned, identical git failure => byte-equal. The real
block's byte-stability is coding_context's own contract; this test is
about plugin sections.

tests/agent/test_plugin_prompt_sections.py

@@ -42,6 +42,16 @@ def _install_test_section(manager: PluginManager, content) -> None:
 
 
 def test_real_aiagent_builds_section_once_and_keeps_it_out_of_static_prefix(monkeypatch):
+    # Pin the workspace snapshot: build_coding_workspace_block shells out to
+    # live `git status`/`git log` on every build, and a git call failing or
+    # timing out under xdist contention makes the two builds differ in the
+    # Branch/Recent-commits lines — a flake unrelated to what this test
+    # asserts (plugin sections). Byte-stability of the REAL workspace block
+    # is coding_context's contract, covered by its own tests.
+    monkeypatch.setattr(
+        "agent.coding_context.build_coding_workspace_block",
+        lambda cwd=None: "Workspace (snapshot at session start):\n- Root: /pinned",
+    )
     calls = []
 
     def section(session_info):