fix(litellm): defer import until model usage

[OiPunk]

Summary

This defers litellm import in litellm_model.py so importing agents.extensions.models.litellm_model does not eagerly import LiteLLM.

This targets issue #610, where Windows users hit UnicodeDecodeError at module import time.

What changed

• Replaced top-level import litellm with a lazy loader (_import_litellm) and a module proxy.
• Kept the existing dependency error message, but now raised at first actual LiteLLM usage.
• Added regression tests for both paths:
  • deferred import + import failure path (including cached failure)
  • successful import + cached module reuse path

Why this helps

• Importing litellm_model no longer fails immediately due to LiteLLM import-time issues.
• The failure is deferred to actual LiteLLM usage, which avoids breaking unrelated imports and startup paths.

Testing

• uv run --with ruff ruff check src/agents/extensions/models/litellm_model.py tests/models/test_litellm_deferred_import.py tests/models/test_litellm_user_agent.py tests/models/test_map.py
• uv run --with pytest pytest -q tests/models/test_litellm_deferred_import.py tests/models/test_litellm_user_agent.py tests/models/test_map.py
• uv run --with litellm --with pytest pytest -q tests/models/test_kwargs_functionality.py tests/models/test_litellm_extra_body.py
• uv run --with pytest python -m trace --count --coverdir /tmp/openai_agents_610_trace_1770722012 --module pytest tests/models/test_litellm_deferred_import.py -q

Trace output confirms all newly added lazy-import branches were executed.

[OiPunk]

Follow-up fix pushed for CI typecheck failure (new commit: e386968).

Root cause:

• mypy flagged dynamic ModuleType test attributes in test_litellm_deferred_import.py.

What I changed:

• Kept the same test behavior, but switched to explicit Any casts for dynamic module attributes so both ruff and mypy pass.

Local re-validation:

• uv run --with ruff ruff check tests/models/test_litellm_deferred_import.py
• uv run --with pytest pytest -q tests/models/test_litellm_deferred_import.py tests/models/test_litellm_user_agent.py tests/models/test_map.py
• uv run --with litellm --with pytest pytest -q tests/models/test_kwargs_functionality.py tests/models/test_litellm_extra_body.py
• uv run --with litellm --with mypy mypy --follow-imports=silent tests/models/test_litellm_deferred_import.py