Add MCP client Streamable HTTP transport

[pushpak1300]

Currently the MCP client can only talk to servers over stdio (Client::local()), so there's no way to connect to a remote or hosted MCP server over HTTP.

This adds a Streamable HTTP transport (MCP 2025-11-25):

// before: stdio only
$client = Client::local('npx', ['-y', '@modelcontextprotocol/server-everything']);

// after: connect over HTTP
$client = Client::web('https://mcp.example.com/mcp');
$client->tools();

// with a static bearer token
Client::web('https://mcp.example.com/mcp')->withToken(env('REMOTE_TOKEN'));

It POSTs each JSON-RPC frame via the Http facade (so it's Http::fake() testable), handles both application/json and text/event-stream responses, captures and echoes MCP-Session-Id, sends MCP-Protocol-Version, re-initializes on a 404, and sends a best-effort DELETE on disconnect. withToken() lives on a new WebClient subclass since stdio clients have no auth concept. No OAuth yet, that's a later stage.

Known limitation: SSE responses are read in full before parsing rather than incrementally, so a server that sends a blocking server->client request mid-stream (say a ping it waits on) would hang until the timeout. Narrow for this client since it advertises no sampling/roots/elicitation capabilities, but I'd like to follow up with incremental streaming. Open to feedback on whether that should block this PR.

Stacked on #226.

[pushpak1300]

Ready for review. Putting in draft until #226 is merged.

[WendellAdriel]

LGTM 🚀

[TitasGailius]

LGTM, great job! 🔥