fix: resolve callable API keys in auxiliary custom endpoint (#79121)

[RelaxJonh]

Summary

vision_analyze crashes with AttributeError: 'function' object has no attribute 'strip' when the main runtime uses a callable api_key (short-lived credential supplier) with a custom OpenAI-compatible endpoint.

Root Cause

The auxiliary client's custom-endpoint branch in resolve_provider_client() calls .strip() directly on explicit_api_key without resolving callables first. The runtime path (line 3069) already preserves callables with api_key if callable(api_key), but the auxiliary resolution path crashes when it encounters a callable.

Two code paths affected:

1. explicit_base_url path (line 5953): (explicit_api_key or "").strip() — callable is truthy, so callable or "" returns the callable, then .strip() raises AttributeError
2. main_runtime fallback path (line 5977): str(main_runtime.get("api_key") or "").strip() — str(callable) returns "<function ...>", which is a string but wrong as an API key

Fix

Resolve callable API keys before string operations in both paths:

• Call the callable and normalize the result to a stripped string
• Consistent with the runtime path's existing callable handling pattern

Test Plan

• Syntax check passes
• Callable explicit_api_key with custom endpoint resolves correctly
• String explicit_api_key continues to work unchanged
• main_runtime with callable api_key resolves correctly

Fixes #79121

[samrusani]

I validated this exact head (6843d07fec) with focused callable-key coverage and prepared a test-only commit on top of it: ff85c31b36a891303c675447d5f34ae35eb38fd7. It covers the async vision path with a callable explicit key and the adjacent callable main_runtime path, asserting one evaluation, stripped token forwarding, and no callable object or function representation reaching client construction. Exact-head result: 17 passed; Ruff and git diff --check also pass. Feel free to cherry-pick ff85c31b36.

[baglett]

Withdrawing this PR — closing as part of a cleanup pass on our submitted contributions.

[baglett]

Withdrawing this PR as part of a contribution cleanup pass.