fix(integrations): Record GitLab webhook Http404 as halt, not error

[sentry]

GitLab webhooks, particularly when an unrecognized project ID or missing author email is encountered, intentionally raise `django.http.Http404`. Previously, the `IntegrationEventLifecycle` wrapper treated these expected 404s as unhandled exceptions, leading to a high volume of spurious Sentry error issues.

An initial attempt to fix this involved adding a blanket Http404 exemption to IntegrationEventLifecycle.__exit__. However, this was identified as problematic, as other integration flows (e.g., identity linking) also re-raise IntegrityError as Http404, and silencing these would mask genuine application errors.

This revised fix addresses the issue by:

1. Reverting the generic Http404 handling from IntegrationEventLifecycle.__exit__ in src/sentry/integrations/utils/metrics.py.
2. Implementing a targeted try/except Http404 block around the event_handler call within the GitLab webhook dispatch loop in src/sentry/integrations/gitlab/webhooks.py. When an Http404 is caught in this specific context, lifecycle.record_halt(e) is explicitly called before re-raising the exception.

This ensures that expected Http404s originating specifically from GitLab webhooks are correctly recorded as halt events (preventing Sentry error issues), while maintaining proper error reporting for Http404s that might indicate genuine problems in other integration contexts.

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Fixes SENTRY-43KZ

_{Comment @sentry <feedback> on this PR to have Autofix iterate on the changes.}