[Client] Implement SSE reconnect with Last-Event-ID

[soyuka]

Context

Per the MCP Streamable HTTP transport spec, when an SSE stream disconnects the client SHOULD reconnect using Last-Event-ID to resume the stream without losing notifications. src/Client/Transport/HttpTransport.php::processSSEStream currently abandons the stream on EOF.

Scope

• Track the most recent id: line per active stream.
• On unexpected EOF (stream still expected open — pending request unresolved, or session active), reconnect with:
  • GET <endpoint> + Accept: text/event-stream
  • Mcp-Session-Id header
  • Last-Event-ID: <id> header
• Configurable retry policy: backoff (1s, 2s, 4s, capped at 10s) + max attempts (default 5).
• Respect retry: SSE field when present.

Conformance scenarios unblocked

sse-retry.

Dependencies

Independent — can run in parallel with auth work.

Acceptance

• Unit tests for ID tracking + reconnect with Last-Event-ID.
• Conformance: sse-retry passes.

cc @soyuka