test(doctor): cover custom provider config regression

[kilhyeonjun]

Summary

• add a config-level regression test for custom providers declared in opencode.json
• verify checkConfig() does not report Model override uses unavailable provider for a valid custom provider such as kiro
• link the regression coverage to the published false-positive reported in bug(doctor): released binary still flags custom providers as unavailable #3293

Validation

• bun test src/cli/doctor/checks/config.test.ts src/cli/doctor/checks/model-resolution-cache.test.ts src/cli/doctor/checks/model-resolution.test.ts --bail

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Summary by cubic

Add a regression test to ensure custom providers declared in opencode.json are recognized by the doctor config check and not flagged as unavailable. Verifies checkConfig() accepts a valid provider like kiro, guarding against the false positive reported in #3293.

^{Written for commit 02d64e4. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 1 file

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Auto-approved: Only adds a regression test case to an existing test file. No production code changes, so zero risk of functional regressions.}

[kilhyeonjun]

Clarifying the intent of this PR since my previous comment was formatted poorly:

This PR is only for regression coverage, not for reintroducing the doctor fix itself.

The core fix already landed on dev in #3200, but the released v3.16.0 binary still reproduces the false-positive reported in #3293. This test makes sure checkConfig() continues to treat custom providers declared in opencode.json as valid going forward.