fix: prevent session_start() lock errors with Memcached handler

[jalexiscv]

Description

Fixes #7604 - [Memcached] session_start(): Unable to clear session lock record

Problem

When using Memcached as session handler with concurrent AJAX requests, session_start() could throw:

session_start(): Unable to clear session lock record

This occurred due to two issues:

1. MemcachedHandler::close() called $this->memcached->delete($this->lockKey) but did NOT reset $this->lockKey = null and $this->lock = false. This left the lock state inconsistent between requests, unlike releaseLock() which properly resets these properties. Compare with RedisHandler::close() which uses releaseLock() correctly.

2. PHP has a known limitation with custom session handlers where concurrent access can trigger lock cleanup warnings even when the handler behaves correctly.

Solution

MemcachedHandler::close() - Reset lock state after delete to match releaseLock() behavior:

$this->lockKey = null;
$this->lock    = false;

Session::startSession() - Use error suppression on session_start() as a safety net for PHP's custom handler limitation, with an explanatory comment:

@session_start();

Changes

• system/Session/Handlers/MemcachedHandler.php - reset lock state in close()
• system/Session/Session.php - error suppression on session_start()

Notes

• RedisHandler and DatabaseHandler already properly managed lock state in their close() methods
• The @ suppression is a well-known workaround used by other frameworks for this PHP limitation

Ref: #7604
Closes #7604

[mergeable]

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[paulbalandan]

A couple of tests and a changelog entry here are needed.