fix: preserve optional string JSON arguments

[pragnyanramtha]

Summary

• skip JSON pre-parsing for simple scalar union annotations such as str | None
• keep JSON pre-parsing for structured optional types such as list[str] | None
• add FastMCP metadata regressions for optional string and optional list arguments

Fixes #1873.

Context

FastMCP already preserves JSON-looking values for plain str arguments, but str | None still went through json.loads(...) during argument pre-parsing. That means an optional string receiving a JSON object or array string was converted to a dict/list before Pydantic validation, then rejected or corrupted instead of reaching the tool as the annotated string value.

This keeps structured annotations parseable while treating unions of simple scalar types like simple scalar fields.

Validation

• uv run --frozen pytest tests/server/mcpserver/test_func_metadata.py -> 36 passed
• uv run --frozen pytest tests/server/mcpserver/test_tool_manager.py -> 50 passed
• uv run --frozen ruff check src/mcp/server/mcpserver/utilities/func_metadata.py tests/server/mcpserver/test_func_metadata.py -> passed
• uv run --frozen pyright src/mcp/server/mcpserver/utilities/func_metadata.py tests/server/mcpserver/test_func_metadata.py -> 0 errors
• git diff --check -> passed

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