fix(config): allow user-defined providers to override built-in providers

[heathdutton]

Fixes #8240

When users define a model provider in config.toml with the same name as a built-in provider (e.g., ollama), the user's configuration should override the built-in defaults. Previously, or_insert was used which only inserted if the key didn't exist, causing user-defined base_url values to be ignored for built-in provider names.

Changed to insert so user-defined providers properly override built-in ones.

Added a regression test to verify the fix.

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[heathdutton]

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