test: stop testing mutable data — convert change-detectors to invariants (#13363)

Catalog snapshots, config version literals, and enumeration counts are data
that changes as designed. Tests that assert on those values add no
behavioral coverage — they just break CI on every routine update and cost
engineering time to 'fix.'

Replace with invariants where one exists, delete where none does.

Deleted (pure snapshots):
- TestMinimaxModelCatalog (3 tests): 'MiniMax-M2.7 in models' et al
- TestGeminiModelCatalog: 'gemini-2.5-pro in models', 'gemini-3.x in models'
- test_browser_camofox_state::test_config_version_matches_current_schema
  (docstring literally said it would break on unrelated bumps)

Relaxed (keep plumbing check, drop snapshot):
- Xiaomi / Arcee / Kimi moonshot / Kimi coding / HuggingFace static lists:
  now assert 'provider exists and has >= 1 entry' instead of specific names
- HuggingFace main/models.py consistency test: drop 'len >= 6' floor

Dynamicized (follow source, not a literal):
- 3x test_config.py migration tests: raw['_config_version'] ==
  DEFAULT_CONFIG['_config_version'] instead of hardcoded 21

Fixed stale tests against intentional behavior changes:
- test_insights::test_gateway_format_hides_cost: name matches new behavior
  (no dollar figures); remove contradicting '$' in text assertion
- test_config::prefers_api_then_url_then_base_url: flipped per PR #9332;
  rename + update to base_url > url > api
- test_anthropic_adapter: relax assert_called_once() (xdist-flaky) to
  assert called — contract is 'credential flowed through'
- test_interrupt_propagation: add provider/model/_base_url to bare-agent
  fixture so the stale-timeout code path resolves

Fixed stale integration tests against opt-in plugin gate:
- transform_tool_result + transform_terminal_output: write plugins.enabled
  allow-list to config.yaml and reset the plugin manager singleton

Source fix (real consistency invariant):
- agent/model_metadata.py: add moonshotai/Kimi-K2.6 context length
  (262144, same as K2.5). test_model_metadata_has_context_lengths was
  correctly catching the gap.

Policy:
- AGENTS.md Testing section: new subsection 'Don't write change-detector
  tests' with do/don't examples. Reviewers should reject catalog-snapshot
  assertions in new tests.

Covers every test that failed on the last completed main CI run
(24703345583) except test_modal_sandbox_fixes::test_terminal_tool_present
+ test_terminal_and_file_toolsets_resolve_all_tools, which now pass both
alone and with the full tests/tools/ directory (xdist ordering flake that
resolved itself).

tests/tools/test_terminal_output_transform_hook.py

@@ -173,6 +173,8 @@ def test_terminal_output_transform_does_not_change_approval_or_exit_code_meaning
 
 
 def test_terminal_output_transform_integration_with_real_plugin(monkeypatch, tmp_path):
+    import yaml
+
     hermes_home = Path(os.environ["HERMES_HOME"])
     plugins_dir = hermes_home / "plugins"
     plugin_dir = plugins_dir / "terminal_transform"
@@ -184,7 +186,15 @@ def test_terminal_output_transform_integration_with_real_plugin(monkeypatch, tmp
         'lambda **kw: "PLUGIN-HEAD\\n" + kw["output"] + "\\nPLUGIN-TAIL")\n',
         encoding="utf-8",
     )
+    # Plugins are opt-in — must be listed in plugins.enabled to load.
+    cfg_path = hermes_home / "config.yaml"
+    cfg_path.write_text(
+        yaml.safe_dump({"plugins": {"enabled": ["terminal_transform"]}}),
+        encoding="utf-8",
+    )
 
+    # Force a fresh plugin manager so the new config is picked up.
+    plugins_mod._plugin_manager = plugins_mod.PluginManager()
     plugins_mod.discover_plugins()
 
     long_output = "X" * 60000