Port LSP client extension from TypeScript to Rust

[SSFSKIM]

Summary

This PR ports Claude Code's Language Server Protocol (LSP) client stack from TypeScript to Rust, enabling Codex to drive language servers for code intelligence features (go-to-definition, find-references, hover, diagnostics, etc.).

The implementation mirrors the three-layer architecture from Claude Code Src/:

1. Transport layer (transport.rs): Async JSON-RPC framer over child process stdio with LSP Content-Length framing
2. Instance layer (instance.rs): Per-server lifecycle management (spawn → initialize → request/notify with transient-error retry)
3. Manager layer (manager.rs): Routing by file extension, document sync, and diagnostic registry
4. Tool layer (tool.rs): Model-facing lsp/query tool exposing nine operations (go-to-definition, find-references, hover, etc.)

Key Changes

New Crates & Modules

• codex-lsp crate: Complete LSP client implementation with ~1,500 lines of core logic
  • transport.rs (403 lines): JSON-RPC framer with request/response routing and notification dispatch
  • instance.rs (305 lines): Server lifecycle with initialize handshake and transient-error retry (ContentModified backoff)
  • manager.rs (363 lines): Extension-based routing, document sync, diagnostic batching
  • tool.rs (394 lines): Nine LSP operations exposed as a single namespaced tool
  • format.rs (289 lines): Human-readable formatting of LSP results (locations, diagnostics, symbols)
  • diagnostics.rs (219 lines): Passive registry with deduplication, severity sorting, and volume capping
  • config.rs (88 lines): Validated runtime view of server configs
  • schema.rs (37 lines): JSON schema generation for tool input
  • extension.rs (118 lines): Extension wiring (thread-scoped manager, tool registration, diagnostic injection)

Configuration

• codex-config::LspServerConfig (config/src/lsp_config.rs): New TOML config struct for [lsp_servers.<name>] table
  • Mirrors [mcp_servers] shape: command, args, env, extension-to-language mapping, workspace folder, initialization options, timeouts, restart limits
  • Generated JSON schema in core/config.schema.json

Integration Points

• ApplyPatchDocSync trait (extension-api/src/apply_patch_doc_sync.rs): Seam between apply-patch runtime and LSP manager for document sync without creating a dependency cycle
• lsp_sync.rs in codex-core: Forwards committed file changes to the LSP manager via the trait object
• Extension registration in app-server/src/extensions.rs: Installs the LSP extension alongside other extensions

Testing

• Unit tests for all layers:
  • transport_tests.rs: JSON-RPC framer over tokio::io::duplex
  • instance_tests.rs: Server lifecycle and handshake
  • manager_tests.rs: Extension routing and document sync
  • tool_tests.rs: Tool dispatch and operation formatting
  • diagnostics_tests.rs: Registry deduplication and volume capping
  • config_tests.rs: Config validation and normalization
  • format_tests.rs: Location and diagnostic formatting
  • extension_tests.rs: Extension wiring
• Smoke test (smoke_tests.rs): Real rust-analyzer integration (ignored by default, requires rust-analyzer on PATH)
• Test support (test_support.rs): Shared helpers for framing and mock server scripting

Design Notes

• Lazy server start: Servers are not spawned until first use; process exit is observed implicitly (stream closure fails in-flight requests with JsonRpcError::Closed)
• Position encoding: Wire uses UTF-16 (declared in initialize), tool input/output uses

https://claude.ai/code/session_01HHRZTCpEbspDRhizowVDFT