fix(models): repair persisted mod handles on startup

[frezbo]

Local agents can load persisted model handles before provider mods register. Older normalization can prepend the provider twice, causing cold-start model resolution to target a nonexistent provider/model until the user reselects the model.

Preserve valid mod handles and collapse one duplicated provider prefix both before and after mod registration. Keep built-in provider aliases on their existing canonicalization path.

👾 Generated with Letta Code

Summary

• Preserve persisted custom-provider model handles when local agents start before their provider mod is registered.
• Collapse duplicated provider prefixes both before and after provider registration.
• Keep built-in provider aliases on the existing canonicalization path.

Without this fix, a persisted handle such as chatgpt-oss/chatgpt-oss/gpt-5.6-sol targets a nonexistent model until the user reselects it through /model.

Verification

• bun test src/backend/local/local-model-normalization.test.ts
  • 9 tests passed
  • 57 assertions passed
• bun run check
  • 12 checks passed

AI Disclosure

• This pull request was written with AI assistance and reviewed and edited by a human
• I have read the AI Policy and agree to its terms

AI Tool(s) Used

Letta Code and GPT Sol 5.6

Human Verification

I have reviewed and understand every change in this pull request and take responsibility for its correctness.

[Copilot]

Pull request overview

Fixes local-agent cold starts where persisted custom-provider model handles can be normalized into a double-prefixed form before a provider mod has registered, leading to an unresolvable provider/model until the user reselects /model.

Changes:

• Avoids re-prefixing persisted model handles when model_settings.provider_type is not a built-in provider type and the handle is already correctly prefixed.
• Collapses a single duplicated provider prefix for unregistered (custom/mod) provider types (e.g. x/x/model → x/model).
• Adds regression tests covering pre-mod-load preservation/repair and ensuring built-in provider alias canonicalization is unchanged.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/backend/local/local-model-normalization.ts	Adds special-case handling to preserve or de-duplicate custom/mod provider prefixes before mod registration, while keeping built-in canonicalization behavior.
src/backend/local/local-model-normalization.test.ts	Adds tests for pre-registration mod handles and for duplicated-prefix repair, plus a guard test for built-in provider alias canonicalization.

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