fix: add mergeToolResultText for Mistral/Devstral models in OpenRouter

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Related GitHub Issue

Closes: #10618

Related: #10540 (same root cause, different provider)

Description

This PR attempts to address Issue #10618. Feedback and guidance are welcome.

Root Cause:
The error "Unexpected role 'user' after role 'tool'" occurs because Mistral/Devstral models have strict message ordering requirements. After a tool role message, the next message cannot be a user role message.

Solution:

1. Extended model detection to include "devstral" (the model mistralai/devstral-2512 contains "devstral" not "mistral")
2. Added mergeToolResultText: true to the conversion options, which merges text content (like environment_details) into the last tool message instead of creating a separate user message

This uses the same mergeToolResultText mechanism that PR #10598 exposes as a setting for OpenAI Compatible providers, but applies it automatically for Mistral family models in OpenRouter since we can detect them by model ID.

Test Procedure

1. Configure OpenRouter with a Mistral/Devstral model (e.g., mistralai/devstral-2512)
2. Start a task that uses tools
3. Verify that tool calls complete without the "Unexpected role user after role tool" error
4. The existing mergeToolResultText tests in openai-format.spec.ts cover the underlying functionality

Pre-Submission Checklist

• Issue Linked: This PR is linked to an approved GitHub Issue (see "Related GitHub Issue" above).
• Scope: My changes are focused on the linked issue (one major feature/fix per PR).
• Self-Review: I have performed a thorough self-review of my code.
• Testing: The underlying mergeToolResultText functionality is already tested.
• Documentation Impact: I have considered if my changes require documentation updates (see "Documentation Updates" section below).
• Contribution Guidelines: I have read and agree to the Contributor Guidelines.

Documentation Updates

• No documentation updates are required.

Additional Notes

This is a minimal fix that leverages existing infrastructure. The mergeToolResultText option was added in a previous PR and is already used by DeepSeek, Z.ai, and MiniMax providers for similar message ordering issues.

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Important

Extends model detection and uses mergeToolResultText for Mistral/Devstral models in OpenRouterHandler to fix message ordering errors.

• Behavior:
  • Extends model detection to include "devstral" in OpenRouterHandler in openrouter.ts.
  • Adds mergeToolResultText: true to convertToOpenAiMessages() for Mistral/Devstral models to prevent "Unexpected role 'user' after role 'tool'" errors.
• Misc:
  • Uses existing mergeToolResultText mechanism from PR feat: add strict tool message ordering setting for OpenAI Compatible providers #10598 for Mistral family models.

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Review complete. No issues found.

The change correctly extends Mistral model detection to include Devstral models and adds mergeToolResultText: true to prevent the "Unexpected role 'user' after role 'tool'" error. The fix leverages existing tested infrastructure.

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