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

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

src/Client/Transport/HttpTransport.php::processSSEStream から始め、既存の SSE 解析とストリーム状態の処理を追跡します。イベント ID の追跡と再接続についてユニットテストのカバレッジを追加し、その後 sse-retry 適合シナリオを実行します。再接続が Last-Event-ID を使用して再開され、retry ポリシーが守られれば完了です。