Fix state isolation bug - eliminate shared singleton

[27Bslash6]

Summary

• Fixes race conditions caused by shared singleton SequentialThinkingServer instance
• Creates new instance per request for complete state isolation
• Adds comprehensive concurrency tests

The Problem

Single SequentialThinkingServer instance at index.ts:128 was shared across ALL requests:

const thinkingServer = new SequentialThinkingServer();

This caused:

• Race conditions in concurrent requests
• Session pollution (different conversations mixing thoughts)
• Incorrect thoughtHistoryLength values
• Non-deterministic behavior

The Fix

Moved instance creation inside the request handler:

server.setRequestHandler(CallToolRequestSchema, async (request) => {
  if (request.params.name === "sequentialthinking") {
    const thinkingServer = new SequentialThinkingServer();
    return thinkingServer.processThought(request.params.arguments);
  }
  // ...
});

Each request now gets a fresh instance with isolated state.

Tests Added

Three new tests in lib.test.ts:

1. State isolation between different server instances
2. Branch isolation (no cross-contamination)
3. Interleaved concurrent request handling

Test Results

All 40 tests pass:

• 37 existing tests (unchanged)
• 3 new concurrency tests

Impact

• Zero breaking changes
• No API modifications
• Stateless architecture (correct for MCP servers)
• No memory leaks (instances are GC'd after request)

Resolves #1

[27Bslash6]

Closing - this was meant for my fork, not upstream. Apologies for the noise.