fix: include tool_choice in ChatCompletionCache cache key (issue #7968)

[truongsontung]

Summary

Fixes #7968.

ChatCompletionCache._check_cache computed the cache key from messages, tools, json_output, and extra_create_args — but ignored tool_choice. Two requests that differed only in tool_choice ("auto" vs "required" vs "none") therefore collided on the same cache key, and a stale cached response was served for the wrong tool_choice.

Before

r1 = await client.create(msgs, tool_choice="auto")      # cached (call #1)
r2 = await client.create(msgs, tool_choice="required")  # served r1 from cache — WRONG

After

• tool_choice is now passed into _check_cache and included in the serialized cache-key payload (normalized: Tool → .schema, otherwise the literal string).
• Keys for different tool_choice values are distinct; identical requests still dedupe.

Changes

• python/packages/autogen-ext/src/autogen_ext/models/cache/_chat_completion_cache.py: thread tool_choice through _check_cache and into the cache-key data dict.
• tests/models/test_chat_completion_cache.py: add test_cache_key_includes_tool_choice (verifies auto/required/none produce distinct keys, and that identical tool_choice is stable).

Test plan

• pytest tests/models/test_chat_completion_cache.py → 26 passed, 1 skipped
• Non-breaking: only widens the cache key; previously-cached entries simply miss and re-fetch.

Happy to adjust the normalization if maintainers prefer a different scheme.

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