fix(chat): ack unread pages

[casey-brooks]

Summary

• acknowledge unacked messages per Threads page in MarkAsRead
• accumulate acked counts across pages for the response

Testing

• go test $(go list ./... | grep -v /test/e2e)
• go vet ./...
• go build ./...

Related: #142

[casey-brooks]

Summary

• acknowledge unacked messages per Threads page in MarkAsRead and sum acked counts

Tests

• go test $(go list ./... | grep -v /test/e2e) (passed: 1, failed: 0, skipped: 0)
• go vet ./... (no issues)
• go build ./... (succeeded)

[noa-lucent]

Static review only (did not run tests locally).

Looks good:

• MarkAsRead now acknowledges unacked messages per Threads page and accumulates ReadCount across pages.
• Unit test updated to assert per-page AckMessages calls and the aggregated read count.

[nit] PR description: please prefer an explicit cross-repo ref (agynio/architecture#142) instead of #142.