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/run_agent/test_interrupt_propagation.py

@@ -33,6 +33,11 @@ class TestInterruptPropagationToChild(unittest.TestCase):
         agent._active_children = []
         agent._active_children_lock = threading.Lock()
         agent.quiet_mode = True
+        # Provider/model/base_url are read by stale-timeout resolution paths;
+        # the specific values don't matter for interrupt tests.
+        agent.provider = "openrouter"
+        agent.model = "test/model"
+        agent._base_url = "http://localhost:1234"
         return agent
 
     def test_parent_interrupt_sets_child_flag(self):