Fix unit test hangs in HTTP transport connection attempts

[Copilot]

Unit tests were hanging indefinitely when attempting HTTP transport connections. The SDK's Connect() calls for streamable and SSE transports would block waiting for protocol handshakes that mock servers couldn't provide.

Changes

• Add connection timeouts (internal/mcp/connection.go)

  • Added 5-second timeout contexts to tryStreamableHTTPTransport() and trySSETransport()
  • Enables fail-fast fallback to plain JSON-RPC transport

• Fix test configuration (internal/server/unified_http_backend_test.go)

  • Added dummy header to TestHTTPBackend_SessionIDPropagation
  • Forces plain JSON-RPC transport directly, avoiding incompatible SDK transports

// Before: SDK attempts streamable/SSE transports, hangs on mock server
cfg := &config.Config{
    Servers: map[string]*config.ServerConfig{
        "test-http": {
            Type: "http",
            URL:  mockServer.URL,
        },
    },
}

// After: Headers force plain JSON-RPC, completes immediately
cfg := &config.Config{
    Servers: map[string]*config.ServerConfig{
        "test-http": {
            Type:    "http",
            URL:     mockServer.URL,
            Headers: map[string]string{"X-Test": "test"}, // Forces plain transport
        },
    },
}

Tests now complete in milliseconds instead of timing out.

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