test(anthropic/chat): force PR-local model_cost map via autouse fixture

CI runs without LITELLM_LOCAL_MODEL_COST_MAP=True, so litellm.model_cost
is loaded from main-branch JSON (default model_cost_map_url) instead of
the PR's checked-out model_prices_and_context_window.json. Tests that
assert per-model flags added in this PR (supports_max_reasoning_effort,
supports_xhigh_reasoning_effort) therefore pass locally but fail in CI
with 'AssertionError: assert False is True' on 5 cases:

  - test_anthropic_model_supports_effort_param_recognizes_supporting_models
    [anthropic.claude-mythos-preview, bedrock/.../mythos-preview,
     claude-opus-4-5-20251101]
  - test_supports_effort_level_handles_provider_prefixes
    [bedrock/invoke/us.anthropic.claude-sonnet-4-6-max-True,
     claude-sonnet-4-6-max-True]

Add an autouse fixture at tests/test_litellm/llms/anthropic/chat/conftest.py
that monkey-patches litellm.model_cost to the PR-local JSON for every test
in this directory. The parent conftest already snapshots+restores
litellm.model_cost per-function, so the mutation is contained.

This is a scoped workaround. The proper fix is to set the env var
globally in the test workflow once the ~10 inline self-set test files
are audited; tracking that as a follow-up issue.

tests/test_litellm/llms/anthropic/chat/conftest.py

@@ -0,0 +1,54 @@
+"""
+Local-conftest for ``tests/test_litellm/llms/anthropic/chat``.
+
+Why this exists
+---------------
+``litellm.model_cost`` is loaded once at ``litellm.__init__`` time. By default
+it fetches ``model_prices_and_context_window.json`` from the **main branch on
+GitHub** (``litellm.model_cost_map_url``) — *not* from the PR-branch JSON in
+the working tree. That works fine in production (operators get new models
+without redeploying litellm) but is the wrong default for tests, which need
+to validate the code in front of them against the data in front of them.
+
+Several anthropic chat transformation tests (``test_supports_effort_level_*``,
+``test_anthropic_model_supports_effort_param_*``) assert per-model flags
+like ``supports_max_reasoning_effort`` / ``supports_xhigh_reasoning_effort``
+that may exist in the PR-local JSON but not yet in main's JSON. Without this
+fixture those tests pass locally (where AGENTS.md tells contributors to set
+``LITELLM_LOCAL_MODEL_COST_MAP=True``) but fail in CI (which doesn't set the
+env var) — the chicken-and-egg PR adds flag → CI fetches main without flag
+→ test fails → flag never lands on main.
+
+The fixture force-loads ``litellm.model_cost`` from the local JSON for every
+test in this directory. ``tests/test_litellm/conftest.py`` already snapshots
+and restores ``litellm.model_cost`` per-function, so this mutation is safe
+and contained.
+
+This is a scoped workaround. The proper fix is to set
+``LITELLM_LOCAL_MODEL_COST_MAP=True`` globally in the test workflow once the
+~10 inline-set test files have been audited and the few tests that exercise
+the remote-fetch / integrity-validation path have been given carve-outs.
+Tracked at https://github.com/BerriAI/litellm/issues/27122.
+"""
+
+import os
+
+import pytest
+
+import litellm
+from litellm.litellm_core_utils.get_model_cost_map import get_model_cost_map
+
+
+@pytest.fixture(autouse=True)
+def _use_pr_local_model_cost_map(monkeypatch):
+    """Force ``litellm.model_cost`` to the PR-branch JSON for the duration of
+    each test. ``monkeypatch`` reverts the env var after the test; the parent
+    conftest restores ``litellm.model_cost`` from its snapshot.
+    """
+    monkeypatch.setenv("LITELLM_LOCAL_MODEL_COST_MAP", "True")
+    monkeypatch.setattr(
+        litellm,
+        "model_cost",
+        get_model_cost_map(url=litellm.model_cost_map_url),
+    )
+    yield