fix: resolve Mistral API compatibility issues

[NicolaivdSmagt]

Summary

Fixes multiple Mistral API compatibility issues that occur when using Mistral models with tool calling:

• Message sequence error: Resolves AI_APICallError: Unexpected role 'user' after role 'tool' by inserting assistant messages between tool and user messages to maintain proper conversation flow
• Assistant message format: Fixes Error: Unsupported content type in assistant message: undefined by ensuring inserted messages have proper AI SDK structure
• Tool call ID format: Resolves tool call ID validation errors by normalizing IDs to meet Mistral's alphanumeric 9-character requirement

Root Cause

Mistral API requirements:

1. Conversation roles must strictly alternate user/assistant/user/assistant after tool usage
2. Tool call IDs must be exactly 9 alphanumeric characters
3. Assistant messages require proper structure with toolInvocations field

Solution

Added Mistral-specific transforms in packages/opencode/src/provider/transform.ts:

• normalizeMistralToolCallIds() - converts tool call IDs to 9-character alphanumeric format
• fixMistralMessageSequence() - inserts assistant messages to maintain proper sequence
• Applied transforms for all Mistral models by checking provider ID

Test Plan

• Verify tool call ID normalization to 9-character alphanumeric format
• Verify message sequence correction inserts assistant messages between tool and user
• Verify existing valid sequences remain unchanged
• Test conversations with tool usage using Mistral models
• Covers all Mistral model variants via provider ID detection

Files Changed

• packages/opencode/src/provider/transform.ts - Added Mistral-specific message transforms

Resolves API errors that occur when using Mistral models with tool calling functionality.

closes#526
closes#1680
closes#2865

[uloco]

I don't want to be that guy but can we please get this in? I really want to use mistral instead of copilot with gtp4.1 but I am facing too many issues with it. What is the blocker here?

[ErwanLegrand]

Fixes #526 , #1680 and #2865 .

[rekram1-node]

ill check this out tmr, sorry for delays

[rekram1-node]

thank u!