Ports ext-apps to @modelcontextprotocol/sdk ^2.0.0-alpha.2 using the v2
backwards-compat layer. Source delta is 4 files +44/-31; the compat layer
keeps v1 deep-import paths and the ZodSchema overloads working.

Unavoidable changes:
- Protocol<X,Y,Z> -> Protocol<ContextT> generic-arity (events.ts)
- assertCapability* override params widened to string (app.ts, app-bridge.ts)
- registerAppTool callback unifies LegacyToolCallback | ToolCallback to
  cover both raw-shape and StandardSchema args (server/index.ts)

CI will fail until @modelcontextprotocol/sdk@2.0.0-alpha.2 is published;
see typescript-sdk milestone v2.0.0-bc. package-lock.json intentionally
not updated in this commit.

events.ts: convert property-override arrow functions to method overrides
with declared overload signatures. The overrides now expose only the
ZodSchema form (the only one this package uses); a guard skips tracking
during super() construction (Protocol's ctor registers built-in handlers
before our fields initialize). Adds _methodOf() helper, removes the
MethodSchema local type and the eslint-disable directives.

server/index.ts: split registerAppTool into two public overloads
(StandardSchemaWithJSON and ZodRawShape forms) matching registerTool's,
removing the union-args casts. The impl signature still loses the
overload pairing, so the forwarding call into registerTool uses one
@ts-expect-error in place of three casts. Re-export LegacyToolCallback
alongside ToolCallback.