Implement token exchange strategy and integrate with BackendClient

[jhrozek]

Implement RFC 8693 token exchange strategy for authenticating Virtual MCP Server to backend MCP servers, and wire authentication into the BackendClient request flow.

This addresses phases of the authentication work from epic #150 (in epics repo).

Scope

• Token exchange strategy implementation reusing pkg/auth/tokenexchange
• Integration with BackendClient (CallTool, ReadResource, GetPrompt)
• Request authentication using OutgoingAuthenticator

Acceptance Criteria

• Token exchange strategy implements Strategy interface
• Strategy successfully exchanges vMCP token for backend token
• BackendClient.CallTool integrates authentication
• BackendClient.ReadResource integrates authentication
• BackendClient.GetPrompt integrates authentication
• All tests pass with race detector enabled
• Integration tests prove end-to-end token exchange works