fix: include tool_choice in ChatCompletionCache cache key

[HumphreySun98]

Why are these changes needed?

ChatCompletionCache forwards tool_choice to the underlying client, where it
materially changes the model's output ("auto" vs "required" vs "none" vs a
specific forced Tool). However _check_cache did not include tool_choice
when computing the cache key, so two otherwise-identical calls that differ only
in tool_choice collide on the same key — the second call silently returns the
first call's cached result:

r1 = await cached_client.create(messages, tools=[tool], tool_choice="none")
r2 = await cached_client.create(messages, tools=[tool], tool_choice="required")
# r2 is r1's cached result: no tool call, despite tool_choice="required"

This PR includes tool_choice in the cache-key data (representing a forced
Tool by its schema name) and passes it through from both create and
create_stream. A regression test asserts the four tool_choice forms produce
distinct cache keys and that the default matches explicit "auto".

Note: existing caches are effectively invalidated by the key change (same as any
cache-key fix); entries were ambiguous between tool_choice values anyway.

Related issue number

N/A — found during review of the cache key computation.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/ (none required for this change).
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed (ran ruff, mypy, and the relevant pytest locally).