fix: transaction processors no longer discard each other's results

[jamescrosswell]

SentryClient.CaptureTransaction passed the original transaction to every processor in the chain instead of the running result:

foreach (var processor in scope.GetAllTransactionProcessors())
{
    processedTransaction = processor.DoProcessTransaction(transaction, hint);   // <- transaction, not processedTransaction

ISentryTransactionProcessor explicitly allows returning a new instance — "The transaction returned can be the same instance received or a new one." With more than one processor, every intermediate result was therefore discarded and only the last processor's output survived, while each processor saw the unprocessed original. The result is a partially processed transaction, silently.

Anyone whose processors mutate and return the same instance — the common case — is unaffected, which is why this went unnoticed. SentryEventHelper.ProcessEvent already chains correctly, so the two loops had simply diverged.

Spotted by @ric-oliv while reviewing #5607; pulled out into its own PR so it isn't blocked behind that stack and gets its own changelog entry.

The regression test fails without the one-line fix (the second processor receives the original rather than the first's replacement) and passes with it.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.85%. Comparing base (85cc2bf) to head (4c62a25).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5621   +/-   ##
=======================================
  Coverage   74.85%   74.85%           
=======================================
  Files         515      515           
  Lines       18962    18962           
  Branches     3694     3694           
=======================================
  Hits        14194    14194           
  Misses       3891     3891           
  Partials      877      877           

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