feat: allow overriding CustomField dependencies via dependency_provider

[anzzyspeaksgit]

Summary

This PR adds support for overriding CustomField classes and instances using dependency_provider.override(), addressing #176.

Problem

Closes #176
Currently, users are unable to override CustomFields at runtime using dependency_provider for testing purposes. dependency_provider.override() crashes or has no effect when provided with a CustomField class or instance.

Solution

Modified CallModel._solve and _asolve to intercept evaluations of CustomField and apply mocked overrides from provider.overrides (checked by instance or type).
Updated Provider.override to skip calling build_call_model on CustomField objects to prevent instantiation/signature errors. Type annotations updated to accept CustomField.
Added tests in test_overrides.py to ensure both CustomField classes and specific instances can be safely overridden.

Testing

• Tests pass locally.
• Verified both sync/async paths and field=True/False variants.

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This PR was created with AI assistance.