feat(lsp): implement Phase 2 LSP Client

[bug-ops]

Summary

Complete LSP client implementation with JSON-RPC 2.0 transport layer, enabling communication with language servers.

Components Implemented

• Transport Layer (lsp/transport.rs)

  • Header-content message format parsing (Content-Length)
  • Async stdio communication with BufReader/BufWriter
  • Message framing and buffering

• LSP Client Core (lsp/client.rs)

  • Concurrent request handling with tokio channels
  • Atomic request ID generation
  • Background message loop with tokio::select!
  • Timeout support for all requests

• Lifecycle Management (lsp/lifecycle.rs)

  • Server spawning with tokio::process
  • Initialize/initialized handshake
  • Capability negotiation (position encoding)
  • Graceful shutdown sequence

• JSON-RPC Types (lsp/types.rs)

  • Request, Response, Notification types
  • RequestId enum (Number/String)
  • Proper serde serialization

• Position Encoding (bridge/encoding.rs)

  • MCP (1-based) ↔ LSP (0-based) conversion
  • UTF-8/UTF-16/UTF-32 encoding support
  • Multibyte character handling (emoji, CJK)

Quality Reviews

Review	Status	Details
Performance	✅ APPROVED	Grade B+, excellent async patterns
Security	✅ APPROVED	LOW risk, no unsafe code
Testing	✅ APPROVED	25 tests passing
Code Review	✅ APPROVED	No blocking issues

Test Plan

• All 25 unit tests pass (cargo test)
• No clippy errors (cargo clippy)
• Code compiles on stable Rust
• Position encoding roundtrip tests
• JSON-RPC serialization tests

Changes

• 8 files changed, 1072 insertions(+), 87 deletions(-)
• New file: crates/mcpls-core/src/lsp/types.rs

Next Steps

After merge, Phase 3 (Document State Management) will implement:

• Document tracking with didOpen/didChange/didClose
• Version management
• LSP synchronization