fix(copilot): extend mini model exclusion to cover gpt-5.x-mini variants

[Juhwa-Lee1023]

Issue for this PR

Closes #15823

Type of change

• Bug fix

What does this PR do?

shouldUseCopilotResponsesApi in packages/opencode/src/provider/provider.ts excluded only models whose ID starts with gpt-5-mini. This means minor-versioned mini models like gpt-5.1-mini or gpt-5.2-mini were incorrectly routed to the Responses API instead of the Chat Completions API, since their IDs do not start with gpt-5-mini.

The fix broadens the exclusion: any model in the gpt-5 family whose ID contains mini is now excluded from the Responses API. This matches the original intent from PR #5877 (exclude all lightweight/mini variants) while staying forward-compatible with OpenAI's major.minor naming convention.

// Before
function shouldUseCopilotResponsesApi(modelID: string): boolean {
  return isGpt5OrLater(modelID) && !modelID.startsWith("gpt-5-mini")
}

// After
function shouldUseCopilotResponsesApi(modelID: string): boolean {
  if (!isGpt5OrLater(modelID)) return false
  if (modelID.startsWith("gpt-5") && modelID.includes("mini")) return false
  return true
}

Model ID	Before	After
gpt-5-mini	✅ Chat API	✅ Chat API
gpt-5.1-mini	❌ Responses API (wrong)	✅ Chat API
gpt-5.2-mini	❌ Responses API (wrong)	✅ Chat API
gpt-5	✅ Responses API	✅ Responses API
gpt-5.1	✅ Responses API	✅ Responses API

How did you verify your code works?

Reviewed the existing shouldUseCopilotResponsesApi unit tests in packages/opencode/test/provider/provider.test.ts and confirmed all pass with the updated logic.

Screenshots / recordings

N/A — logic-only change, no UI.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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