[BACKPORT 2025.2][#31220] CDC: Add cleanup_stale_cdc_streams yb-admin command (#32025)

[egladysh]

Summary

Adds a CleanupStaleCDCStreams RPC to the MasterReplication master
service and a corresponding cleanup_stale_cdc_streams yb-admin
command.

The command scans the cdc_state table and identifies stale entries —
rows whose CDC stream no longer exists, or whose tablet no longer
exists. It supports:

• A dry_run flag to report stale entries without deleting them.

Example (dry run):

./bin/yb-admin \
    cleanup_stale_cdc_streams dry-run

Found 2 stale cdc_state entries (dry run).
  tablet_id: 8f3b9b3a8b2c4e2b9c7d6a5f4e3d2c1b, stream_id: 3d9f1e2a4b6c4d8e9f0123456789abcd, table_id: 000033e8000030008000000000004000, table_name: orders, reason: stream not found
  tablet_id: missing_tablet_id, stream_id: 7a8b9c0d1e2f34567890abcdef123456, reason: tablet not found

Implementation notes:

• The cdc_state table is scanned once and materialized so both the
  tablet-collection and classification loops share one consistent
  snapshot.
• Tablet table metadata is resolved under a single SharedLock before
  the classification loop to avoid repeated lock acquisitions.
• xCluster streams that carry an empty namespace_id have their
  namespace resolved via their first table_id.

Upgrade/Rollback safety

The proto change is purely additive: new messages
(CleanupStaleCDCStreamsRequestPB, CleanupStaleCDCStreamsResponsePB)
and a new RPC method (CleanupStaleCDCStreams) are added to
MasterReplication. No existing message fields are modified.

On a mixed-version cluster, an older master that does not have this RPC
will return UNIMPLEMENTED when cleanup_stale_cdc_streams is invoked;
all other CDC and xCluster operations are unaffected.

Rolling back removes the yb-admin command and the RPC handler. No
on-disk state, catalog schema, or gflag defaults are changed.

Original commit: c02cbb5 / #32025

Test plan

• CDCServiceTest.TestCleanupStaleCDCStreamsWithoutCDCStateTable —
  verifies OBJECT_NOT_FOUND when the cdc_state table does not exist.
• CDCServiceTest.TestCleanupStaleCDCStreamsDryRunAndDelete —
  dry-run returns stale entries without deleting; live run deletes exactly
  those entries and leaves the valid entry untouched.
• CDCServiceTest.TestCleanupStaleCDCStreamsNamespaceFilter — with
  a namespace filter, only entries attributable to the selected namespace
  are deleted; cross-namespace and unattributable (both stream and tablet
  missing) entries are preserved.

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Co-authored-by: Sumukh-Phalgaonkar 61342752+Sumukh-Phalgaonkar@users.noreply.github.com

[egladysh]

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