fix: configuration merging for built-in model providers

[otterammo]

Changes

These changes seek to resolve this issue.

Core

• mod.rs
  • Updated the configuration merging logic to use insert instead of or_insert. This ensures that user-defined providers in config.toml correctly override the built-in defaults.

Verification

I have added a reproduction test case test_user_config_overrides_builtin_provider to core/src/config/mod.rs

• This test verifies that a user-defined ollama provider in config.toml correctly overrides the built-in one.

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[otterammo]

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