fix(activesync): separate PING watermark from SYNC modseq to stop iOS…

[TDannhauer]

Summary

Fixes ~3-second PING loops on iOS devices with many IMAP folders. Two separate
failure modes were sharing the same state logic:

1. MODSEQ drift: PING compared against the SYNC modseq but intentionally
   did not advance it, so the same change was re-reported every few seconds.
2. sync_pending during PING: An interrupted SYNC with <MoreAvailable />
   left a UID batch in sync_pending. PING treated that batch as a change
   signal, never called ping(), and could not persist a PING checkpoint.

The fix is a three-state model with deliberately separate code paths:

State	Purpose	Storage
SYNC watermark	CHANGEDSINCE	$_status / modseq()
PING watermark	"client notified?"	$_pingStatus / ps in sync_data
SYNC pending	MOREAVAILABLE batch	sync_pending (SYNC only)

All EAS Protocol versions are affected handling emails via imap

Changes

Core logic

• Folder/Imap.php: $_pingStatus, pingModseq(), acknowledgePingStatus();
  optional ps key in sync_data serialization
• Imap/Adapter.php: ping() compares against the PING watermark, not SYNC modseq()
• State/Base.php: documented SYNC vs. PING paths in getChanges(); ignore
  sync_pending when ping=true; persist PING checkpoint via
  save(['preservePending' => true])
• State/Sql.php / State/Mongo.php: preservePending keeps sync_pending
  intact via _syncPendingBlob

Documentation

• Architecture comments at the central decision points (getChanges(), $_pingStatus,
  sync_pending column, preservePending)
• Cross-references between Base, Folder, and Adapter for overall context

Tests

• ImapFolderTest — PING checkpoint, serialization, stale SYNC modseq scenario
• ImapAdapterTest — PING uses pingModseq(), not SYNC modseq()
• StateSqlPreservePendingTest — preservePending does not wipe sync_pending

Verification

Tested against a production iOS device with 100+ mail folders:

Metric	Before	After
Found changes! bursts	~12,200 / ~3s interval	None
False PING folder responses	Every ~3s	Stopped
ps in sync_data	Missing for affected collections	Present
Heartbeat behavior	Immediate re-response	Normal Sleeping for N seconds

Note: Collections with an open sync_pending batch from an earlier interrupted
SYNC are expected to retain that state. PING no longer loops on it; the client must
complete a SYNC for those collections to drain the batch.

Test plan

• phpunit — ImapFolderTest, ImapAdapterTest, StateSqlPreservePendingTest
• Device log: no Found changes! burst, no false PING folder responses
• DB: ps present in sync_data for affected collections; sync_pending preserved on PING save
• Manual: observe iOS mail client for 10–15 minutes under normal use
• Optional: force SYNC on a collection with open sync_pending → batch clears after SYNC completes

[ralflang]

Please rebase before adding code to this. I will put it back into draft status now.