Fix Mistral model matching bug - change from "tral-" to "mistral-" prefix

[Copilot]

The Mistral provider's matchesModel() method was incorrectly using model.startsWith("tral-") instead of model.startsWith("mistral-"), causing it to fail to match valid Mistral model names like "mistral-small-latest".

Problem

• Models like "mistral-small-latest" were not being matched by the Mistral provider
• The method was checking for the wrong prefix: "tral-" instead of "mistral-"
• This meant requests with Mistral models would fail to route to the correct provider

Solution

Changed the model matching logic from:

return model.startsWith("tral-")

To:

return model.startsWith("mistral-")

Validation

• ✅ "mistral-small-latest" now correctly matches
• ✅ "mistral-large" now correctly matches
• ✅ Invalid models like "tral-test" correctly don't match
• ✅ Follows the same pattern as other providers (OpenAI uses "gpt-", Anthropic uses "claude-", etc.)
• ✅ Build passes successfully

Fixes #5.

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