feat: Add BeforeSendFeedback callback to inspect, modify, or drop user feedback before it's sent

[vladbrincoveanu]

Refers to #4092

Summary

Adds a BeforeSendFeedback callback to SentryOptions, mirroring the existing BeforeSend pattern but scoped to user feedback events. It lets applications inspect, modify, or drop user feedback before it is sent to Sentry.

Changes

• Two SetBeforeSendFeedback overloads on SentryOptions (with and without SentryHint), consistent with SetBeforeSend / SetBeforeSendTransaction.
• SentryEventHelper.DoBeforeSendFeedback, invoked in SentryClient.CaptureFeedback after event processors run.
• New CaptureFeedbackResult.DroppedByBeforeSendFeedback result value
• Dropped feedback is recorded via ClientReportRecorder under DiscardReason.BeforeSend / DataCategory.Feedback. Originally, was specified the use of DataCategory USER_REPORT_V2.
  [SDK docs] add new UF data model and implementation details under telemetry dev docs sentry#88232 (comment)

Behavior notes for reviewers

Assumption: Feedback is a common place to scrub PII, so a callback that cannot complete should not leak partially-scrubbed or unintended feedback.
Implementation: Both the null-return and throw paths record a client-report discard. This is documented on the public API and covered by tests.

Tests

SentryClientTests covers: callback invocation + mutation passthrough, hint received from caller, null and throw drops feedback.

Unrelated changes

Added a small unit test for CaptureFeedbackResult.UnknownError.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.19%. Comparing base (4b51c11) to head (7bbbb1c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5361      +/-   ##
==========================================
+ Coverage   74.14%   74.19%   +0.04%     
==========================================
  Files         509      509              
  Lines       18386    18409      +23     
  Branches     3600     3603       +3     
==========================================
+ Hits        13633    13658      +25     
+ Misses       3880     3879       -1     
+ Partials      873      872       -1     

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