Enhancement: Support Remote Auth for Docker Containers

Enhancement: Support Remote Auth (--remote-auth-*) for Docker Containers

Summary

Enable --remote-auth-* flags to work with containerized MCP servers, allowing ToolHive to acquire OAuth/OIDC tokens and inject them into requests to Docker containers, not just remote URLs.

Current Behavior

The --remote-auth-* flags currently only work when connecting to remote MCP servers via HTTP/HTTPS URLs. When running containerized MCP servers locally, remote authentication is disabled by design.

Current limitation in pkg/transport/http.go:314:

// Add OAuth token injection middleware for remote authentication if we have a token source
if t.remoteURL != "" && t.tokenSource != nil {
    tokenMiddleware := t.createTokenInjectionMiddleware()
    middlewares = append(middlewares, types.NamedMiddleware{
        Name:     "oauth-token-injection",
        Function: tokenMiddleware,
    })
}

The check t.remoteURL != "" prevents token injection for containers since remoteURL is only set for remote URLs, not container images.

Desired Behavior

Allow users to acquire OAuth/OIDC tokens via the --remote-auth-* flags and have ToolHive inject those tokens into HTTP requests to containerized MCP servers running with HTTP/SSE transport.

Example command:

thv run ghcr.io/example/github-mcp-server \
  --transport sse \
  --remote-auth-issuer https://github.com/login/oauth \
  --remote-auth-client-id <client-id> \
  --remote-auth-scopes "repo,user"

Expected flow:

1. ToolHive initiates OAuth flow with GitHub
2. User authenticates in browser
3. ToolHive acquires access token
4. Container starts with HTTP/SSE transport
5. ToolHive injects Authorization: Bearer <token> into all requests to the container
6. Container can use the token to make authenticated GitHub API calls

Start in pkg/transport/http.go:314 and trace how remoteURL and tokenSource control OAuth token injection for HTTP/SSE transports. Verify the container startup and transport path, then ensure the acquired Bearer token is injected into requests made to containerized MCP servers when the remote-auth flags are used.