Fix(chat): prompt caching feature flag enrollement (#7177) (#7188)

This commit fixes a bug in the prompt caching feature flag
implementation where we were incorrectly using the enrollment event
result instead of the actual feature flag value. This caused prompt
caching to be enabled for all users regardless of their feature flag
state.

Key changes:
- Added proper feature flag value tracking in PromptBuilder using a
PromptCachingSetting interface
- Separated the enrollment event logging from the feature flag
evaluation
- Ensures the feature flag value is correctly respected while still
tracking enrollment events

The fix maintains proper A/B testing capabilities while correctly
respecting the intended feature flag distribution for prompt caching
functionality.

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prompt caching should not be used for users who are not enrolled
(non-dot com users)

(cherry picked from commit 9cc2415)


## Test plan

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Co-authored-by: Beatrix <[email protected]>