Fix MCPEXP002 constant mismatch causing macOS Debug build failure

[Copilot]

The build (macos-latest, Debug) workflow failed with CS0117 due to mixed usage of Subclassing_* and Extensibility_* MCPEXP002 constants after the rename. This PR reconciles the constant surface and updates remaining stale call sites.

• Compatibility constants for MCPEXP002

  • Added Extensibility_DiagnosticId and Extensibility_Url as aliases in Experimentals to keep both naming paths valid during transition.
  • Added Extensibility_Message for the renamed extensibility terminology.

• Updated stale attribute call sites

  • Switched [Experimental(...)] usage from Subclassing_* to Extensibility_* in:
    • McpServerRequestHandler.RoutingNameParameter
    • JsonRpcMessageContext.RoutingName

• Resulting pattern

  [Experimental(Experimentals.Extensibility_DiagnosticId, UrlFormat = Experimentals.Extensibility_Url)]
  public string? RoutingName { get; set; }

This keeps MCPEXP002 behavior unchanged while removing the source of compile-time symbol mismatches in CI.

[jeffhandley]

I intended to iterate on #1739 rather than creating a fresh PR.