[INF-1722] Keep single deletes on primary key

[thesyncim]

Ticket

• https://linear.app/stream/issue/INF-1722

Summary

Remove redundant equal lower and upper composite-row bounds from single-row primary-key DELETEs. In the live c3 target, those bounds made PostgreSQL choose the 39 GB channels_app_pk_created_at_cid_retention_idx instead of the unique channels_pkey, producing a roughly 4.5 minute delete outlier.

Exact scalar equalities remain in catalog primary-key order, so row identity and delete semantics are unchanged while the planner can select the unique primary key.

Validation

• go test ./...
• go vet ./...
• go test -race ./internal/cdc
• Live read-only EXPLAIN: bounded predicate selected the retention index; exact PK equalities selected channels_pkey

Rollout

• The target-only retention index is temporarily deferred during c3 catchup.
• Do not deploy this follow-up image during catchup without explicit approval.
• Before cutover, deploy the fix, recreate the deferred target index concurrently in follow mode, and verify it is ready/valid and replay remains current.
• No source schema or row changes.

Checklist

• The changed code has been covered with unit tests
• No API endpoints changed
• No documentation change is required