Use Redis-native atomic ops for concurrency counter instead of Go mutex

[Hariprasath8064]

Right now the concurrent request counter is protected with a process-local sync.Mutex in redisDB.go. This fixes race conditions inside a single process but serializes all access from that process and does not provide atomicity across multiple processes/containers using the same Redis key. We should replace the mutex-based approach with a Redis-native atomic solution (e.g., a small Lua script or atomic INCR/DECR pattern that enforces the concurrency limit server-side), so that the "check + increment" for the concurrency limit is performed atomically in Redis and scales correctly across all load tester instances.

[Parthivsurya]

/assign

[Devpool-soc]

Alright @Parthivsurya! Consider yourself officially assigned as now. If you are thinking, "Whoa, hold up, I didn't sign up for this," no worries, just yell "/unassign" and we can pretend this never happened. But if you are sticking around, just know your clock for this ticks out on: 2026-01-12. Don't screw it up :)

[Parthivsurya]

@Naganathan05 @Hariprasath8064

[Hariprasath8064]

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