feat(agent-detail): group model selectors by provider

[binsarjr]

Summary

• group Agent Detail primary model options in native HTML <optgroup> elements by provider
• apply the same provider grouping to the fallback model selector
• retain the existing global-default and no-fallback options outside model groups

Verification

• git diff --check

The selector grouping follows the native <optgroup> pattern: each provider is a group label, and its configured models are the options inside it.

[anvie]

@copilot resolve the merge conflicts in this pull request

[anvie]

@binsarjr bisa tolong diresolve dulu conflictnya?

[binsarjr]

oke, sudah ya pak @anvie