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).

AGENTS.md

@@ -566,3 +566,52 @@ python -m pytest tests/ -q -n 4
 Worker count above 4 will surface test-ordering flakes that CI never sees.
 
 Always run the full suite before pushing changes.
+
+### Don't write change-detector tests
+
+A test is a **change-detector** if it fails whenever data that is **expected
+to change** gets updated — model catalogs, config version numbers,
+enumeration counts, hardcoded lists of provider models. These tests add no
+behavioral coverage; they just guarantee that routine source updates break
+CI and cost engineering time to "fix."
+
+**Do not write:**
+
+```python
+# catalog snapshot — breaks every model release
+assert "gemini-2.5-pro" in _PROVIDER_MODELS["gemini"]
+assert "MiniMax-M2.7" in models
+
+# config version literal — breaks every schema bump
+assert DEFAULT_CONFIG["_config_version"] == 21
+
+# enumeration count — breaks every time a skill/provider is added
+assert len(_PROVIDER_MODELS["huggingface"]) == 8
+```
+
+**Do write:**
+
+```python
+# behavior: does the catalog plumbing work at all?
+assert "gemini" in _PROVIDER_MODELS
+assert len(_PROVIDER_MODELS["gemini"]) >= 1
+
+# behavior: does migration bump the user's version to current latest?
+assert raw["_config_version"] == DEFAULT_CONFIG["_config_version"]
+
+# invariant: no plan-only model leaks into the legacy list
+assert not (set(moonshot_models) & coding_plan_only_models)
+
+# invariant: every model in the catalog has a context-length entry
+for m in _PROVIDER_MODELS["huggingface"]:
+    assert m.lower() in DEFAULT_CONTEXT_LENGTHS_LOWER
+```
+
+The rule: if the test reads like a snapshot of current data, delete it. If
+it reads like a contract about how two pieces of data must relate, keep it.
+When a PR adds a new provider/model and you want a test, make the test
+assert the relationship (e.g. "catalog entries all have context lengths"),
+not the specific names.
+
+Reviewers should reject new change-detector tests; authors should convert
+them into invariants before re-requesting review.