docs: update MCP contribution setup

[oleksii-tumanov]

Summary

• Update Playwright MCP source paths in CONTRIBUTING.md to point at the current Playwright monorepo locations.
• Update focused MCP test commands to the current package scripts: ctest-mcp and test-mcp.

Context

• fix(test-mcp): fix test runner mcp issues, cover with tests playwright#37365 renamed the focused MCP package scripts from mcp-test / mcp-ctest to test-mcp / ctest-mcp.
• chore: extract src/tools playwright#39632 moved the MCP implementation into the current packages/playwright-core/src/tools/... layout.

Testing

• git diff --check