Feat: Http SSE transport as per '2024-11-05' MCP spec

[vinitkadam03]

Description

This PR adds support for the HTTP with Server-Sent Events (SSE) transport protocol, as defined in the MCP specification (2024-11-05). This enables the Relay package to connect to MCP servers that expose HTTP+SSE endpoints, complementing the existing Http and Stdio transports.

The implementation follows the MCP HTTP+SSE transport flow:

1. Opens a persistent SSE connection to the server's SSE endpoint
2. Reads the initial endpoint event to obtain the message posting URL with session ID
3. Sends initialize request with required params (protocolVersion, capabilities, clientInfo)
4. Sends initialized notification after successful initialization
5. Sends JSON-RPC requests via HTTP POST and receives responses via SSE message events

Configuration Example

// config/relay.php
'servers' => [
    'http-sse-mcp' => [
        'transport' => Transport::HttpSse,
        'url' => env('MCP_URL', 'https://mcp.modulcontextprotocol.io/sse'),
        'timeout' => 60,
        'api_key' => env('RELAY_ANALYTICS_SERVER_API_KEY'),
        'headers' => [],
    ],
],

Files Changed

• src/Enums/Transport.php - Added HttpSse enum case
• src/Transport/HttpSseTransport.php - New transport implementation
• src/Transport/TransportFactory.php - Added factory support for HttpSse
• tests/Unit/Transport/HttpSseTransportTest.php - Test suite (19 test cases)
• tests/TestDoubles/HttpSseTransportFake.php - Test double for mocking
• tests/Unit/Transport/TransportFactoryTest.php - Added test case for HttpSse factory creation

Testing

All existing tests pass (78 tests, 170 assertions). The new transport includes 19 dedicated test cases.

Breaking Changes

None. This is a purely additive feature that introduces a new transport option without modifying existing transports or APIs.