Cleanup/tool schema 1848

[ShashwatXD]

Problem

base_model_to_openai_function emitted the legacy {"function_call", "functions"} envelope, and base_model_to_dict immediately unpacked functions[0] and rewrapped it as {"type": "function", "function": ...}. Same peel happened again in multi_base_model_to_openai_function.

Changes

• base_model_to_openai_function now returns {"type": "function", "function": {"name", "description", "parameters"}} directly
• base_model_to_dict is a thin wrapper around that (validation only)
• multi_base_model_to_openai_function returns a list of those schemas (was a {function_call, functions} dict — no in-tree callers)
• remove dead *args/**kwargs from base_model_to_dict (forwarding always raised; B026)

Tests

• updated name assertion for the modern shape
• added test_no_legacy_envelope_round_trip (both paths return the same dict, no envelope keys)
• updated test_base_model_to_dict for the modern shape

Closes #1848

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