fix(config): preserve React language IDs for TSX/JSX

[dergachoff]

Summary

Fixes #148.

This keeps React document language IDs intact for TSX/JSX files and still lets the common single TypeScript or JavaScript server configuration handle them.

• typescript + .tsx file patterns now map to typescriptreact
• javascript + .jsx file patterns now map to javascriptreact
• when no exact react LSP client is configured, typescriptreact routes to the typescript server and javascriptreact routes to the javascript server
• if an exact react server is configured, it wins over the fallback

The existing C/C++ file-pattern override behavior is unchanged.

Validation

• cargo +nightly fmt --check
• cargo +1.95 clippy -p mcpls-core --all-targets --all-features -- -D warnings
• cargo +1.95 test -p mcpls-core
• cargo +1.95 nextest run -p mcpls-core
• cargo +1.95 test -p mcpls-core get_client_for_file
• Built a local release binary from a combined branch containing this PR plus fix(lsp): omit null diagnostic request fields #147, then ran a standalone MCP stdio probe against a real TSX project using tsgo. get_diagnostics returned empty diagnostics for both a TSX source file and a TSX test file, with no typescriptreact no-server error and no identifier:null protocol error.