fix(instructor): forward base_url and api_key to instructor.from_provider

[greysonlalonde]

Closes #5204

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Note

Medium Risk
Touches LLM client instantiation and credential propagation (api_key/base_url), which can affect runtime connectivity and authentication across providers. Change is small and guarded (only passes non-None values), with explicit unit coverage.

Overview
InternalInstructor._create_instructor_client now propagates base_url and api_key (when set on a non-string LLM) into instructor.from_provider, enabling custom endpoints/credentials to be honored.

Tests were updated to include base_url/api_key on the unsupported-provider mock and to add assertions that these fields are forwarded when provided and omitted when None.

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[iris-clawd]

LGTM — forwards base_url/api_key to instructor.from_provider when set, clean and backward-compatible.