BFCL/openai: harden msgspec adapter tests + hoist imports + gc docstring for tool_choice

Follow-up to PR #346 (thread https://github.com/mlcommons/endpoints/pull/346#discussion_r3532030052).

In tests/unit/openai/test_msgspec_adapter.py and openai/types.py (@viraatc):

1. Hoist the in-function imports (ModelParams etc.) to module top per AGENTS.md no-lazy-imports rule.
2. Cover the untested wire-format changes with small encode-and-assert tests: stream is now omitted when streaming is off (previously "stream": false), tool_choice encoding has no test, and nothing pins that seed=None is omitted from the encoded payload per omit_defaults.
3. In openai/types.py, add tool_choice to the gc=False AT-RISK mutable-field list in the ChatCompletionRequest docstring (it can hold a dict).