Fix: Matching built-in providers fails if specified by name (Fixes #335)

[ambicuity]

Description

This PR fixes Issue #335 where specifying a built-in provider by name (e.g., provider="google") would fail in configurations using schema constraints. This happened because _create_model_with_schema was attempting to resolve the provider before ensuring that built-in providers were loaded into the registry.

The fix ensures that providers.load_builtins_once() is called unconditionally at the start of _create_model_with_schema.

Fixes #335

Bug fix

How Has This Been Tested?

I verified the fix with a reproduction script that attempted to create a model with provider="gemini" and schema constraints.
Before the fix, this resulted in an InferenceConfigError.
After the fix, the provider is successfully resolved.

Checklist:

• I have read and acknowledged Google's Open Source Code of conduct.
• I have read the Contributing page, and I either signed the Google Individual CLA or am covered by my company's Corporate CLA.
• I have discussed my proposed solution with code owners in the linked issue(s) and we have agreed upon the general approach.
• I have made any needed documentation changes, or noted in the linked issue(s) that documentation elsewhere needs updating.
• I have added tests, or I have ensured existing tests cover the changes
• I have followed Google's Python Style Guide and ran pylint over the affected code.

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