fix(server): use configured model for one-shot text-gen fallback

[macodev00]

What

• One-shot text generation fallback now uses the provider instance’s configured defaultModelSelection or first custom model instead of always hardcoding claude-haiku-4-5.

Why

Fixes #12651

Redo of closed #12732 addressing Macroscope Not-approved (product-default model selection): change stays local to the fallback model pick on the existing instance path; product slugs remain last-resort when nothing is configured.

UI

No UI. Server one-shot commit/PR/branch/title generation model selection only.

Checklist

• I read CONTRIBUTING.md and kept this small / bug-only
• What + why explained above
• Followed AGENTS.md documentation rules (no new docs)
• No unrelated changes
• CLA / AI-reviewed standing override for macodev00 auto-contrib

Summary by CodeRabbit

• Bug Fixes
  • Improved text-generation fallback behavior when a provider instance is disabled.
  • Fallbacks now honor configured instance defaults, custom model settings, and legacy custom model configurations.
  • Ensures the appropriate configured model is selected instead of an unintended default.

[macroscopeapp]

Approvability

Verdict: Approved at 9fe1e58

Macroscope's review found this PR approvable — This is a localized server bug fix that makes one-shot text generation respect existing configured model selections and custom models, with targeted regression coverage. Existing fallback defaults remain intact when no configured model is available.

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ℹ️ Recent review info

⚙️ Run configuration

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Review profile: CHILL

Plan: Advanced

Run ID: 47468ebf-7239-452a-9e3c-f84588be5426

📥 Commits

Reviewing files that changed from the base of the PR and between 1de563c and 9fe1e58.

📒 Files selected for processing (2)

• apps/server/src/serverSettings.test.ts
• apps/server/src/serverSettings.ts

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📝 Walkthrough

Walkthrough

The server now resolves text-generation fallbacks from matching default selections and configured custom models before using provider defaults. Tests cover instance and legacy custom model configurations.

Changes

Text generation fallback

Layer / File(s)	Summary
Fallback resolution logic apps/server/src/serverSettings.ts	The fallback resolver reuses a matching defaultModelSelection, checks instance and legacy custom models, and then uses the existing default model maps.
Fallback behavior tests apps/server/src/serverSettings.test.ts	Tests verify fallback selection for a configured default model, an instance custom model, and a legacy custom model. They also verify the resolved claude-haiku-4-5 model in the disabled-provider case.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~15 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: juliusmarminge

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description check	✅ Passed	The description explains the change, motivation, scope, and checklist status. It covers the required template content, although some section headings differ slightly from the template.
Title check	✅ Passed	The title clearly and concisely describes the main change: using a configured model for one-shot text-generation fallback.
Linked Issues check	✅ Passed	The PR implements the fallback requirement in [#12651] and [#12732]. fallbackTextGenerationProvider first uses the matching defaultModelSelection, then the first configured custom model from the p…
Out of Scope Changes check	✅ Passed	The reviewed changes are limited to apps/server/src/serverSettings.ts and its tests. They change fallback model selection and add regression coverage for the linked issue. No unrelated UI or product…

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