fix: share task store across stateless HTTP requests in serve()

[bokelley]

Summary

• serve() created a new InMemoryTaskStore per HTTP request, so tasks/get on subsequent requests returned "Task not found" — breaking all async tool calls over stateless Streamable HTTP
• Now serve() creates one shared task store and passes it to the factory via a new ServeContext parameter
• Backward compatible: existing no-arg factories still work (JS ignores extra arguments)
• Updated examples, docs (BUILD-AN-AGENT.md, llms.txt), and exports

Closes #442

Test plan

• 7 new integration tests in test/lib/serve-tasks.test.js:
  • Sync tool over stateless HTTP
  • Async task lifecycle (create → poll → result) over stateless HTTP
  • Custom taskStore via ServeOptions
  • Same taskStore reference across all requests
  • Regression callToolStream task retrieval fails over stateless Streamable HTTP #442: task created on connection A retrievable on connection B
  • Backward compat: no-arg factory still works
• All 8 existing serve.test.js tests pass
• All 6 existing mcp-tasks-compat.test.js tests pass
• Full suite: 1,499 tests pass, 0 failures

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