Fix #596: Qwen integration: qwen3.5-plus unsupported and OAuth setup ...

[Zakir Jiwani (JiwaniZakir)]

Closes #596

Removes qwen3.5-plus from the CUSTOM_PROVIDER_MODELS list in models.ts because the model is unsupported by the Qwen Code OAuth provider and causes 15005ms timeout errors. The three remaining entries (coder-model, qwen3-coder-plus, qwen3-coder-flash) are all confirmed working with the OAuth flow.

• entrypoints/app/ai-settings/models.ts: deleted the { modelId: 'qwen3.5-plus', contextLength: 1000000 } entry from the qwen-code block in CUSTOM_PROVIDER_MODELS
• entrypoints/app/ai-settings/models.test.ts (new): adds getModelsForProvider('qwen-code') unit tests asserting the three valid model IDs are present and qwen3.5-plus is absent

Verified by the new Bun test suite, which explicitly asserts ids does not contain 'qwen3.5-plus' and does contain 'coder-model', 'qwen3-coder-plus', and 'qwen3-coder-flash'.

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This PR was created with AI assistance (Claude). The changes were reviewed by quality gates and a critic model before submission.

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