fix: prevent race condition during configuration updates

[wasim-builds]

Fixes race condition during concurrent provider updates.

Changes made:

• Wrapped the runtime handlers in a handlerWrapper struct to ensure consistent types for atomic.Value.
• Verified safety of s.mu.Lock() around router rebuild logic.
• Tests pass cleanly under go test -race.

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[wasim-builds]

Hi! The race condition fix is ready for review. The change wraps runtime handlers in a handlerWrapper struct to ensure atomic configuration updates during concurrent provider switches. Tests pass cleanly under go test -race. Happy to make any adjustments!

[wasim-builds]

Quick summary of the fix: the race condition happened because atomic.Value was being used to store interface values of different concrete types (http.Handler vs handlerWrapper). During concurrent config reloads, a goroutine could read a partially-written value. The fix is a single typed wrapper struct so every Store/Load operates on the same concrete type. The change is minimal and only touches the handler storage path — no router logic changed.