mcp: refine extension API (#246856)

- Replace SSE with HTTP in the API (although this still supports the
  same SSE fallback we otherwise have, this is not documented)
- Add a new `resolveMcpServerDefinition` step that is only run before
  the MCP server is started -- both from a cache or from a running
  extension. This lets extensions do user interaction is a more correct way.
- Add appropriate docs and such.

src/vs/workbench/api/common/extHost.api.impl.ts

@@ -1830,7 +1830,7 @@ export function createApiFactoryAndRegisterActors(accessor: ServicesAccessor): I
 			AISearchKeyword: AISearchKeyword,
 			TextSearchCompleteMessageTypeNew: TextSearchCompleteMessageType,
 			ChatErrorLevel: extHostTypes.ChatErrorLevel,
-			McpSSEServerDefinition: extHostTypes.McpSSEServerDefinition,
+			McpHttpServerDefinition: extHostTypes.McpHttpServerDefinition,
 			McpStdioServerDefinition: extHostTypes.McpStdioServerDefinition,
 		};
 	};