Add server lifecycle events (ServerStarted / ServerStopped)

Problem

The SDK currently dispatches only request-scoped events (RequestEvent, ResponseEvent, ErrorEvent, list-changed events). There is no hook that fires once per Server::run() invocation.

Consumers that need one-time setup/teardown per server boot must either:

• Duplicate the logic in every transport's entry point (HTTP controller, CLI command, custom transports), or
• Subscribe to RequestEvent + matching ResponseEvent/ErrorEvent and deal with per-request overhead, fiber suspension, and every error path.

Proposal

Add two events in Mcp\Event:

• ServerStartedEvent — dispatched at the top of Server::run($transport), before the first request is read. Exposes the server and transport.
• ServerStoppedEvent — dispatched just before Server::run() returns, in a finally so it fires on clean exit, thrown exceptions, and transport close. Exposes the exit code and any captured throwable.

Use cases

• Identity / impersonation: switch the current user of a host framework (Drupal, Symfony Security) once when the server boots. This is our concrete motivation — without a lifecycle hook we either patch every transport or pay the cost of switching on every RequestEvent.
• Metrics / observability: increment mcp_server_started_total, start a run-duration timer, emit a "server up" log line including registered tool/prompt/resource counts.
• Resource management: warm caches, acquire leases, open long-lived connections at start; release them at stop.

Alternatives considered

• Use RequestEvent as a pseudo-start hook — works but is per-request. Adds overhead to what is logically a one-shot, and requires pairing with ResponseEvent/ErrorEvent for cleanup. Fiber suspension (Protocol::handleRequest, early return on $fiber->isSuspended()) means the terminal event can be delayed or skipped from the subscriber's perspective.
• Subclassing Server — not portable; transports and framework integrations instantiate Server directly via the Builder.
• Transport-level hooks — would require a change in every transport implementation (Stdio, StreamableHttp, any custom transport) rather than in one place in Server::run().

Backward compatibility

Purely additive. Subscribers that don't care about the new events are unaffected. No public API changes to existing events or handlers.

Server::run($transport) から始め、次に Protocol::handleRequest と Builder によって作成された transport のエントリーポイントを確認します。既存の Events と、それらの dispatching のテストがあれば確認します。ServerStartedEvent が最初のリクエストの前に一度だけ発火し、ServerStoppedEvent が正常終了時、例外発生時、transport のクローズ時に finally から発火し、指定されたコンテキストを保持することが完了条件です。