Add tool call support to Mistral structured handler

[CamilleScholtz]

The Mistral structured handler currently has no tool call support and uses the legacy json_object response format with a system prompt instruction for schema enforcement. This PR adds full tool calling with a two-phase approach and upgrades to json_schema mode.

The problem

Mistral does not allow response_format and tools in the same API request. The current structured handler has no tool support at all, it simply appends a system message with the JSON schema and uses json_object mode.

The solution

A two-phase approach:

1. Phase 1 (tool calling): When tools are present, send requests with tools/tool_choice but without response_format. Handle tool calls in a loop until the model stops or max steps is exhausted.

2. Phase 2 (structured output): Send a final request without tools but with json_schema response format for strict server-side schema enforcement.

Edge cases handled:

• No tools - sends a single request with json_schema response format directly
• Tools present but model doesn't call them - the response has no schema enforcement (since response_format was omitted), so it's discarded and a follow-up request is sent without tools to get proper structured output
• Max steps exhausted - forces a final structured response request

Changes

• Added CallsTools, ExtractsText, ExtractsThinking traits (matching the Text handler)
• Replaced appendMessageForJsonMode() (system prompt hack) with json_schema response format (strict server-side enforcement)
• Added sendAndRespond() orchestration with handleToolCalls() / handleStop() / sendStructuredResponse()
• Extracted addStep() for reuse across tool call and stop flows (matching Text handler pattern)
• Included rateLimits in Meta via processRateLimits() (was missing)
• Included additionalContent from extractThinking() (was missing)

How it follows the existing patterns

This implementation mirrors the Mistral Text handler's structure:

• Same trait usage (CallsTools, ExtractsText, ExtractsThinking, MapsFinishReason, ProcessRateLimits, ValidatesResponse)
• Same addStep() reusable method pattern
• Same shouldContinue() logic (including maxSteps === 0 as infinite)
• Same mapToolCalls() implementation
• Same message sequencing (AssistantMessage → ToolResultMessage → resetToolChoice)