[P3] Integration test for Sentry distributed-trace propagation (flag-gated + target-scoped)

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**File:** `app/scripts/lib/sentry-trace-propagation.ts`, `app/scripts/lib/setupSentry.js`
**Size:** M

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## Problem

PR #42867 (Sentry v8→v10) enabled native W3C `traceparent` propagation (`propagateTraceparent: true`), gated on the `SENTRY_DISTRIBUTED_TRACING_ENABLED` kill switch and scoped to `BACKEND_TRACE_PROPAGATION_TARGETS` (`*.api.cx.metamask.io`). The unit test added for it only asserted that `Sentry.init` received `propagateTraceparent: true` — mock-heavy, high-maintenance, and not covering the behavior that actually matters (flag-gating and target-scoping). It was removed during review.

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## Solution

Add an integration (or e2e) test that drives the real init path and asserts outbound-header behavior, rather than mocking `Sentry.init`.

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## Acceptance Criteria

- [ ] With distributed tracing enabled, outbound requests to backend targets (`*.api.cx.metamask.io`) carry exactly one `traceparent`, alongside `sentry-trace` / `baggage` and the RAPID `baggage` segment.
- [ ] With the kill switch (`SENTRY_DISTRIBUTED_TRACING_DISABLED`), no `traceparent` is injected to those targets.
- [ ] Non-backend hosts never receive `traceparent`.

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## Notes

The #43819 coverage harness already exercises the real init path and captures emitted envelopes; this ticket is specifically for the targeted outbound-header assertions (gating + scoping) that the removed unit test failed to cover.

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## Labels

`team-extension-platform`, `area-sentry`

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## References

- PR #42867 (Sentry v8→v10 upgrade)
- #43819 (coverage-equivalence harness)

Start with app/scripts/lib/sentry-trace-propagation.ts and app/scripts/lib/setupSentry.js, then read the #43819 coverage-equivalence harness to see how the real init path and emitted traffic are exercised. Add targeted outbound-header assertions for enabled and disabled tracing, backend targets, and non-backend hosts; done means the acceptance criteria pass without mocking Sentry.init.

Written by the indexing model from the issue text.