refactor: remove clearContext() from WebMCP polyfill API (#16)

Chrome 147.0.7721.0 is removing provideContext() and clearContext() in favor
of an additive registerTool()/unregisterTool() model. This change removes the
deprecated clearContext() method from our polyfill layer to stay aligned with
the updated W3C specification.

No user-facing impact: the React hook (useMcpTool) never used clearContext()
and continues to use only registerTool()/unregisterTool() for its lifecycle.

Author: kashishhora