Fix Z.AI thinking request serialization

[alejandro-ao]

Summary

• Preserve Z.AI's logical thinking toggle when raw reasoning_effort is unsupported.
• Serialize the documented Z.AI thinking object and only send effort values for explicitly compatible models.
• Add MockTransport payload regressions, provider-config coverage, and protocol documentation.

Verification

• uv sync --dev --locked
• uv run pytest (1878 passed, 3 skipped)
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• Hugo and Pagefind builds
• git diff --check

Fixes #674