Remove AppSec::instrumentation::Gateway#pushed?

[y9v]

What does this PR do?
Moves identity event tracking from the global Gateway singleton to the per-request AppSec Context. Removes Gateway#pushed? and @pushed_events entirely.

The fix adds an identity_event_collected flag to the per-request Context#state hash, set by the identity watchers (watch_user_id, watch_user_login), and read by watch_request_finish. This correctly scopes header collection to the request that triggered the identity event.

Motivation:
Gateway#pushed? tracked events on a process-lifetime singleton that was never reset between requests. Once any request pushed an identity event (e.g. a Devise login), pushed? returned true for all subsequent requests forever. This caused watch_request_finish to collect identity headers on every request after the first login, rather than only on requests where an identity event actually occurred.

Change log entry
None.

Additional Notes:
Related to #5466

How to test the change?
CI and manual testing

[datadog-prod-us1-5]

✅ Tests

🎉 All green!

❄️ No new flaky tests detected
🧪 All tests passed

🎯 Code Coverage (details)
• Patch Coverage: 100.00%
• Overall Coverage: 95.13% (+0.00%)

_{This comment will be updated automatically if new data arrives.
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[pr-commenter]

Benchmarks

Benchmark execution time: 2026-03-23 16:36:12

Comparing candidate commit 39f93b3 in PR branch appsec-remove-pushed-events with baseline commit 8d5d914 in branch master.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 46 metrics, 0 unstable metrics.

Explanation

This is an A/B test comparing a candidate commit's performance against that of a baseline commit. Performance changes are noted in the tables below as:

• 🟩 = significantly better candidate vs. baseline
• 🟥 = significantly worse candidate vs. baseline

We compute a confidence interval (CI) over the relative difference of means between metrics from the candidate and baseline commits, considering the baseline as the reference.

If the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD), the change is considered significant.

Feel free to reach out to #apm-benchmarking-platform on Slack if you have any questions.

More details about the CI and significant changes

You can imagine this CI as a range of values that is likely to contain the true difference of means between the candidate and baseline commits.

CIs of the difference of means are often centered around 0%, because often changes are not that big:

---------------------------------(------|---^--------)-------------------------------->
                              -0.6%    0%  0.3%     +1.2%
                                 |          |        |
         lower bound of the CI --'          |        |
sample mean (center of the CI) -------------'        |
         upper bound of the CI ----------------------'

As described above, a change is considered significant if the CI is entirely outside the configured SIGNIFICANT_IMPACT_THRESHOLD (or the deprecated UNCONFIDENCE_THRESHOLD).

For instance, for an execution time metric, this confidence interval indicates a significantly worse performance:

----------------------------------------|---------|---(---------^---------)---------->
                                       0%        1%  1.3%      2.2%      3.1%
                                                  |   |         |         |
       significant impact threshold --------------'   |         |         |
                      lower bound of CI --------------'         |         |
       sample mean (center of the CI) --------------------------'         |
                      upper bound of CI ----------------------------------'

[Strech]

👍🏼