feat: adopt ModelCapabilitiesOverride for vision-capable model switching (fixes #643)

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Changes

When switching models mid-session via SwitchToAsync, PolyPilot now passes a ModelCapabilitiesOverride instead of null for models that have vision or reasoning capabilities.

What changed

1. ModelCapabilities.GetCapabilitiesOverride(string modelSlug) — new static method that maps the existing ModelCapability flags to an SDK ModelCapabilitiesOverride object:

   • Vision-capable models (e.g. gemini-3-pro): sets Supports.Vision = true and provides Limits.Vision with supported media types (image/png, image/jpeg, image/gif, image/webp), max 10 images, and 20 MB size limit
   • Reasoning-expert models (e.g. claude-opus-4.6, gpt-5): sets Supports.ReasoningEffort = true
   • Models without vision or reasoning flags: returns null (server defaults apply)

2. CopilotService.ChangeModelAsync — now calls GetCapabilitiesOverride(normalizedModel) and passes the result to SwitchToAsync instead of hardcoded null

Why

SDK v0.2.2 added ModelCapabilitiesOverride to ModelApi.SwitchToAsync(). By passing capabilities, the CLI can enforce proper image limits and token budgets per model instead of relying on server defaults. This is especially useful for vision-capable models where image constraints differ.

Testing

• 9 new unit tests in ModelSelectionTests.cs covering all branches of GetCapabilitiesOverride:
  • Vision models get vision support + limits
  • Reasoning models get reasoning effort flag
  • Non-vision/non-reasoning models return null
  • Unknown models return null
  • Models with both flags get both set
• Integration test in ModelCapabilitiesOverrideTests.cs for CI validation
• All 3686 existing tests pass ✅

Fixes #643

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Generated by Agent Fix for issue #643 · ● 23.3M · ◷