The disk-pressure binlog purge only triggers at >=80% usage, which cannot
be safely reached on a large shared host volume in tests (would need a
~177GB fill). Make the threshold overridable via PURGE_BINLOG_DISK_THRESHOLD
(default 80, production behaviour unchanged) so a test can set it below the
current usage and exercise the real purge path without filling the disk.

Refs #3087

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

get_synced_binlogs matched replica status against a reconstructed
${DP_TARGET_POD_NAME}-bin, which fails when the target env drifts from
the actual binlog-writing pod (e.g. after a switchover) - returning
'No synced binlog files found' even when replicas are synced. Match
against LOG_PREFIX (derived from the real log_bin_basename) instead.
Add DEBUG lines printing DP_TARGET_POD_NAME/LOG_PREFIX/replica hosts and
per-replica resolved current_file, so the cleanup-decision path is
observable in the archive-binlog log.

Refs #3087

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

…stderr

get_replica_hosts assumed replicas register report_host, but the semisync
cmpd start command omits --report-host (only the MGR cmpd and
mysql-entrypoint.sh set it). With report_host empty, SHOW REPLICAS leaves the
Host column blank and `awk '{print $2}'` slides onto the Port, returning
"3306" as the replica host (observed: 'Processing replica host: 3306',
'Can't connect to MySQL server on 3306:3306'). Discover connected replicas
from the primary's own Binlog Dump threads instead
(information_schema.PROCESSLIST), which carry the replica ip:port regardless
of report_host; strip the ephemeral :port. Fall back to SHOW REPLICAS / SHOW
SLAVE HOSTS with TAB-split awk so an empty Host stays empty instead of
becoming the Port.

Also move the two DEBUG lines in get_synced_binlogs to stderr: that
function's stdout is captured by `synced_binlogs=$(get_synced_binlogs)`, so
echoing on stdout both hid the lines from the pod log and corrupted the
return value (making the empty result look non-empty and bypassing the
fail-closed 'No synced binlog files found' guard).

Split the `local synced_binlogs=$(...)` declaration from its assignment so
the `$? -ne 0` check reflects get_synced_binlogs' exit code rather than the
always-zero exit of `local`, keeping the fail-closed guard closed.

Refs #3087

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Two data-safety defects found in a deep review of the PITR binlog cleanup.

1. The fail-closed guard `synced_binlogs=$(get_synced_binlogs); if [ $? -ne 0
   ]` is unreachable under the actionset-wide `set -e`: a separated
   `var=$(fn)` assignment where fn returns 1 aborts the whole script before
   the check, so a replica-parse failure crash-loops the archiver instead of
   logging and skipping the purge. (Splitting the earlier `local var=$(...)`
   to fix `$?` masking removed the mask that `set -e` was relying on.) Use
   `if ! synced_binlogs=$(get_synced_binlogs) || [ -z ... ]` so the failure
   is caught without tripping set -e.

2. `min_synced_file` was a leaked global, never reset. On a 2nd+ cleanup call
   in the long-lived archiver, if replica discovery returns empty the loop is
   skipped and the stale previous value passes the guard, purging binlogs on
   an unverified synced position (potential data loss). Declare it `local`
   with an empty reset at function entry.

Both verified: bash confirms the separated assignment aborts under set -e
while `if !` does not; helm render + bash -n pass.