fix(imap_client): load slice data before persisting cache updates

[TDannhauer]

fix(imap_client): load slice data before persisting cache updates

Summary

Fixes PHP 8+ Undefined array key warnings in Horde_Imap_Client_Cache_Backend_Cache::save() when IMP IMAP caching is enabled.

When a cache slice is marked for update, save() re-serializes all UIDs mapped to that slice in the slicemap. Only UIDs touched in the current request are guaranteed to be present in $this->_data[$mbox]. Accessing missing entries (e.g. UID 46751) triggers warnings on PHP 8+.

This change:

• Loads the full slice via _loadSlice() before persisting (consistent with the existing 'add' path)
• Skips and removes stale slicemap entries that have no corresponding cached data

Problem

PHP ERROR: Undefined array key 46751 on line 147/149 of Cache.php

Observed with IMP and 'cache' => 'cache' in backends.local.php.

Solution

foreach (array_keys(array_flip($val['slice'])) as $slice) {
    $this->_loadSlice($mbox, $slice);
    $data = [];
    foreach (array_keys($s['s'], $slice) as $uid) {
        if (!isset($d[$uid])) {
            unset($s['s'][$uid]);
            continue;
        }
        // ...
    }
}

Test plan

• Enable IMAP caching ('cache' => 'cache') in IMP backend config
• Open inbox and read messages that were previously triggering the warning
• Confirm Horde log shows no Undefined array key warnings from Cache.php
• Verify cached message metadata (flags, envelope, etc.) still loads correctly after browsing
• Delete a message and confirm cache updates without warnings
• Optionally flush Memcache and confirm cache rebuilds cleanly on next IMP access

[what-the-diff]

PR Summary

• Improvement on Data Loading
  Added a new functionality that triggers a method _loadSlice($mbox, $slice). This new feature is situated in a process that goes over a collection of 'slices' in the data, making it more efficient in fetching the relevant information.

• Added UID Checks
  Implemented a new safeguard which checks the presence of 'uid' in the data array $d before proceeding. This is done to avoid complications and errors in case 'uid' is not available. If 'uid' is found missing, an operation ensures to clean it from the data set $s['s'][$uid] as well, promoting error-free and seamless processing.