fix: preserve SSE resume cache and correct event replay

[freedom-winds]

Summary

Fix two issues in the Streamable HTTP server's SSE resume behavior:

• The event identified by Last-Event-ID was replayed again.
• Request-scoped SSE caches were deleted immediately after the final response, preventing clients from recovering responses after a disconnect.

Changes

• Adjust the SSE resume offset to replay only events after Last-Event-ID.
• Reject event IDs older than the retained cache window.
• Mark completed request channels as complete while retaining their cached events.
• Allow completed_cache_ttl to evict completed request caches.
• Add regression coverage for retaining completed request channel state.