Fix email callback bundle_version for unpinned Dag runs

[ayanhussain81]

PR #66485 fixed scheduler-emitted TaskCallbackRequests (external kill, heartbeat
timeout, stuck-in-queued) to source bundle_version from dag_run.bundle_version
instead of DagVersion.bundle_version, so that callbacks for Dags with
disable_bundle_versioning=True stay unpinned and run against the same on-disk
code the task did, instead of pinning to a version the run was never pinned to.

That fix touched three call sites but missed a fourth: the EmailRequest built a
few lines below the TaskCallbackRequest in process_executor_events's
external-kill path. It still falls back to DagVersion.bundle_version whenever
dag_version is set, regardless of whether dag_run.bundle_version is None.

For a Dag with disable_bundle_versioning=True and email_on_failure/
email_on_retry configured, when a task is detected as externally killed, the
failure/retry email ends up pinned to a stale bundle version the run was never
pinned to — causing the Dag Processor to check out an unnecessary
versions/<sha>/ working tree for that email callback (the exact class of
problem #66485 set out to fix, just through the one path it didn't touch).

This applies the same guard used at the other three call sites (and used by the
_resolve_ti_callback_bundle_info helper, whose own docstring says
process_executor_events "inlines the same resolution" — this brings it back in
sync), and adds a regression test mirroring the existing
test_external_kill_callback_bundle_version_follows_dag_run for the EmailRequest
path, since the existing TestSchedulerCallbackBundleInfoDagVersionNullable suite
verifies a reimplementation of the logic rather than the real per-callsite code,
which is how this one diverged unnoticed.

Verified locally: the new test fails against the pre-fix code
(AssertionError: -'abc123-sha' +None) and passes with the fix; the full
bundle_version / process_executor_events / heartbeat-timeout test surface in
test_scheduler_job.py passes with no regressions; ruff check and
ruff format --check are clean on both changed files.

---

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Claude Code was used to investigate the root cause (via git blame/git log on the
original fix, and tracing EmailRequest.bundle_version through to
BundleVersionLock), implement the fix, write the regression test, and run/verify
the test suite. All findings and the diff were reviewed and understood before
submission.

---

• Read the Pull Request Guidelines.
• No related issue — this was found via source/history review rather than a bug report.
• Newsfragment ({pr_number}.bugfix.rst) will be added as a follow-up commit once this PR's number is known.

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[bbovenzi]

Closing in favor of #71425 & #73915