fix(mcp): standardize parameter schemas for Google Gemini compatibility

[pcgeek86]

Summary

Fixes #262

When using rustwright-mcp with Google Gemini models via Vertex AI (such as in OpenCode via @ai-sdk/google-vertex), Vertex AI enforces strict validation on OpenAPI 3.03 / JSON Schema tool definitions.

Specifically:

• browser_file_upload.paths previously defined "type": ["array", "null"]. When translated to Gemini's schema format, type: ["array", "null"], it became an anyOf union with items remaining as a sibling field.
• Vertex AI strictly enforces: "When using any_of, it must be the only field set." Having items alongside any_of caused Vertex AI to reject requests with HTTP 400 (Unable to submit request because rustwright_browser_file_upload functionDeclaration parameters.paths schema specified other fields alongside any_of).
• Similarly, browser_network_request.part defined "type": ["string", "null"].
• browser_drag (startElement, endElement), browser_console_messages (filename), browser_network_requests (filter, filename), and browser_handle_dialog (promptText) also used "type": ["string", "null"].

Changes

1. Standardize optional parameter schemas in mcp/src/tools.rs to single types ("type": "array", "type": "string") omitted from "required", aligning with upstream Playwright MCP standards (@modelcontextprotocol/server-playwright).
2. Remove null from browser_network_request.part's enum list.
3. Update unit tests and e2e schema assertions in mcp/src/tools.rs and mcp/tests/stdio_e2e.rs.
4. Update serialized catalog fixtures (mcp/tests/fixtures/tools-list.json, tools-list-lean.json, tools-list-legacy.json) and SHA-256 fixture checksum in mcp/tests/fixtures/README.md.

Verification

• cargo check --locked
• cargo check --manifest-path mcp/Cargo.toml --locked
• cargo test --manifest-path mcp/Cargo.toml --bin rustwright-mcp (all 43 tests pass, including catalog fixture match)
• cargo test --lib (all 397 unit tests pass)