fix: restore double-registration guard on App and AppBridge handlers

The base SDK Protocol silently replaces an existing request or
notification handler. v1's ProtocolWithEvents threw instead, so a stray
setRequestHandler / setNotificationHandler could not disconnect on*
handlers or addEventListener listeners. Restore that guard on both App
and AppBridge with a small MethodRegistry, keep replace semantics for the
on* setters via a protected replaceRequestHandler, and release methods
on removeRequestHandler / removeNotificationHandler.

Also:
- give oncreatesamplingmessage a getter, a replace warning and the same
  clearable shape as the other request setters
- let App register handlers for custom (non-spec) methods; the default
  branch of assertRequestHandlerCapability threw a misleading
  "No handler for method X registered" error
- move the RESOURCE_URI_META_KEY / RESOURCE_MIME_TYPE docs to
  src/constants.ts where the constants now live
- fix stale "inherited from Server" wording and point @see links at
  @modelcontextprotocol/client, which is what hosts install