fix: [Bug] Fix nonce validation race condition due to concurrent PHP session ... - #25124

Fixes #24477

Root cause

DB session save handler overwrites session data with a stale snapshot (last-write-wins)

Changes

• Piwik\Session\SaveHandler\DbTable now remembers the session payload it read at the start of the request and, on write, applies only the values this request actually changed on top of the session data currently stored. Unchanged sessions only refresh modified/lifetime instead of rewriting the data column, changed sessions are stored with a three-way merge (recursive per key, honouring keys the request removed) via a compare-and-swap UPDATE with a bounded retry, and any non-mergeable payload (unparsable data, objects in the session, no read snapshot) falls back to the previous plain write. Regression tests were added to the existing DbTable integration test.