flutter_webmcp 0.3.0

Cancellable WebMCP registration lifecycle for Flutter scopes.

• Adds WebMcpRegistrationAttempt, backed by AbortController in the browser adapter.
• Reconciles scoped registrations independently by tool name.
• Cancels stale registrations before starting same-name replacements.
• Replaces the old asynchronous WebMcpToolRegistrar override with WebMcpToolRegistrationStarter.
• Keeps cleanup errors observable without blocking replacement registrations.

This release contains an intentional breaking API change to WebMcpToolScope; backward compatibility with the old registrar override is not provided.