Preserve nullable types in MCP tool schemas

[Shy7777]

An MCP tool with a field such as {"type": ["string", "null"]} currently disappears from discovery: the schema converter tries to use the list as a dictionary key, and the factory catches the resulting error and skips the tool. This affects both input and typed output schemas. The equivalent anyOf form has a related problem: null falls back to Any, so unrelated values are accepted.

This maps null to NoneType and resolves type arrays as unions, as described in the JSON Schema type reference. Array items use the same conversion path, so nullable items and nested array types are retained. Required fields remain required, with their defaults and descriptions preserved.

Fourteen regression cases fail before the fix. The full core test command now passes with 353 passed and 3 skipped; the skips are existing live MiniMax tests. Tests cover nullable input/output tool discovery, requiredness, defaults, wrong-type rejection, and nested arrays without making provider calls. The repository-wide Black and Flake8 checks pass, along with the pre-commit checks for the changed files.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.