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.

Start at Server::run($transport), then inspect Protocol::handleRequest and the Builder-created transport entry points. Review the existing events and their dispatching tests, if present. Done means ServerStartedEvent fires once before the first request and ServerStoppedEvent fires from finally on clean exit, exceptions, and transport close, carrying the specified context.