[Issue] Fix silent fallback to file sessions when session/save is set to Valkey

[m2-assistant]

This issue is automatically created based on existing pull request: #41099: Fix silent fallback to file sessions when session/save is set to Valkey

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Description (*)

Magento\Framework\Session\SaveHandlerFactory registers only the db and redis session save handlers (app/etc/di.xml). However, bin/magento setup:config:set --session-save=valkey is a documented, supported command that writes session/save = valkey plus a session/valkey connection group to app/etc/env.php.

Because valkey is missing from the handlers map, falls back to the default files handler with no log entry, so sessions are written to var/session even though the store is configured for Valkey. The failure is invisible to operators.

The 2.4.9 Valkey work added Magento\Framework\Cache\Backend\Valkey and the --session-save=valkey / --session-save-valkey-* CLI options, but the matching session SaveHandlerFactory registration was never added.

This PR registers the valkey save handler, reusing the Redis-compatible session handler and reading connection parameters from the session/valkey/session/redis deployment-config group.

Related Pull Requests

None.

Fixed Issues (if relevant)

None.

Manual testing scenarios (*)

1. On a 2.4.9 install with Valkey running, configure Valkey sessions:

bin/magento setup:config:set --session-save=valkey \
--session-save-valkey-host=127.0.0.1 \
--session-save-valkey-port=6379 \
--session-save-valkey-db=2 
bin/magento cache:flush

2. Open the storefront and create a session (add to cart / log in).
3. Before: new files appear under var/session, and valkey-cli -n 2 --scan returns no session keys — silent fallback to files.
4. After: sessions are stored in Valkey — valkey-cli -n 2 --scan returns sess_* keys and valkey-cli -n 2 dbsize grows, while var/session stops growing.
5. Regression: --session-save=redis, --session-save=db, and --session-save=files continue to work unchanged.

Questions or comments

None.

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• README.md files for modified modules are updated and included in the pull request if any README.md predefined sections require an update
• All automated tests passed successfully (all builds are green)