fix(provider): stop restoring cleared custom options

[mimen]

Custom model option descriptors are settings-owned. After a user cleared every descriptor, mergeProviderModels treated the empty list as incomplete provider discovery and restored the previous capabilities. Web and mobile kept showing stale controls until the server restarted.

Custom rows now bypass capability backfill. Built-in rows still retain discovered capabilities across incomplete refreshes.

Verification

• The new regression test failed before the fix because the previous Reasoning descriptor was restored.
• vp test run apps/server/src/provider/Layers/ProviderRegistry.test.ts passes all 48 tests.
• vp run --filter=t3 typecheck passes with existing Effect suggestions only.
• Targeted lint, formatting, and git diff --check pass.

This is a focused follow-up to #9807. #8964 is closed because #9807 superseded its broader custom-model profile design.

Risk and rollback

The change affects only custom rows during provider snapshot merging. Revert af66745a9 to restore the previous behavior.

Review tier: standard.

Implemented with GPT-5.6 Sol via Claude Code.

Note

Fix mergeProviderModels restoring cleared custom options

Custom models in a refreshed provider snapshot now return directly from that snapshot before the existing capability-retention logic runs. Non-custom models still keep previous capabilities when the refreshed model lacks them. Adds a test confirming cleared custom model capabilities stay cleared.

^{Macroscope summarized af66745.}

[macroscopeapp]

Approvability

Verdict: Approved at af66745

Macroscope's review found this PR approvable — The change narrowly fixes provider snapshot merging so explicitly cleared custom model options are not resurrected, while preserving capability fallback for built-in models. It adds a targeted regression test and does not affect schemas, product defaults, or deployment configuration.

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