feat(llm): add MistralAI structured output support

[xxx-0425]

Description

Adds structured output support to MistralAILLM.

This PR:

• Adds the response_format parameter to MistrAILLM.
• Supports Pydantic models using chat.parse and chat.parse_async.
• Supports JSON Schema dictionaries using chat.complete and chat.complete_async.
• Exposes structured output support through supports_structured_output.
• Updates the minimum supported mistralai version to 1.5.0.
• Adds unit tests for synchronous and asynchronous structured output.
• Adds documentation, an example, and a changelog entry.

Fixes #542.

Type of Change

• New feature
• Bug fix
• Breaking change
• Documentation update
• Project configuration change

Complexity

Complexity: Medium

How Has This Been Tested?

• Unit tests
• E2E tests
• Manual tests

Testing performed:

• uv run pytest tests/unit/llm/test_mistralai_llm.py — 29 passed
• Full unit test suite — 1198 passed
• Ruff lint and formatting checks
• MyPy type checking
• Sphinx documentation build
• Package build with uv build
• Manual synchronous and asynchronous tests against the Mistral API

Checklist

• Documentation has been updated
• Unit tests have been updated
• E2E tests have been updated
• Examples have been updated
• New files have copyright header
• CLA (https://neo4j.com/developer/cla/) has been signed
• CHANGELOG.md updated if appropriate