Redis socket tokens can be overwritten under concurrent claim

[anxkhn]

RedisTokenManager.link_token_to_sid currently does a separate exists check and then a set. Two workers can both observe a free token and then overwrite each other, so a client can lose its socket mapping under concurrent reconnects.

Expected: only one worker should own a given token key. The loser should get a fresh token instead of overwriting the winner.

I have a proposed fix in #6771 that uses an atomic SET NX claim with a bounded retry and a local fallback when Redis is unavailable.

Reproduction sketch:

1. Run two app workers against the same Redis.
2. Have two clients claim the same socket token at the same time.
3. Observe one worker overwrite the other's token record.

This is the tracking issue requested on that pull request.

[linear-code]

ENG-11166