De-duplicate callback log reading with the task-instance log reading path

[jason810496]

Depends on #66610 getting merged first.

Summary

PR #66610 added airflow-core/src/airflow/utils/log/callback_log_reader.py to read
deadline-callback execution logs for the UI. Its remote/local read helpers
re-implement logic that already exists in FileTaskHandler
(airflow-core/src/airflow/utils/log/file_task_handler.py) and is fronted by
TaskLogReader (airflow-core/src/airflow/utils/log/log_reader.py). This issue
tracks consolidating the two paths onto shared helpers as a follow-up.

This is tech debt, not a bug — the duplication is intentional in #66610 to keep
that PR focused.

Duplicated logic

callback_log_reader.py	Existing equivalent in file_task_handler.py
_read_callback_remote_logs	FileTaskHandler._read_remote_logs (:967)
_read_callback_local_logs (glob-by-prefix + os.path.commonpath containment)	FileTaskHandler._read_from_local (:862, :880)
remote-first-then-local fallback + _interleave_logs + "Log message source details" group header	FileTaskHandler.read (:737)
tuple[list[str], list[RawLogStream]] return annotation	existing StreamingLogResponse alias

Why a shared abstraction needs design (why it was deferred)

FileTaskHandler's read path is TaskInstance-centric — it takes (ti, try_number, metadata) and renders paths from the TI. Callback logs have no TaskInstance: the
path is a fixed prefix (executor_callbacks/{dag_id}/{run_id}/{callback_id} or
triggerer_callbacks/...). Extracting the common core means separating the
path-resolution step from the storage-read step so both callers can supply their own
relative path(s).

Follow-up work

• Extract the storage-read core (remote-then-local fallback, _interleave_logs,
  source-header emission, os.path.commonpath containment) into a shared helper that
  takes already-resolved relative path(s) rather than a TI.
• Have both FileTaskHandler.read and callback_log_reader.read_callback_log call it.
• Annotate the callback reader helpers with the existing StreamingLogResponse alias.

Acceptance criteria

• callback_log_reader.py no longer carries its own copies of the remote/local read and
  containment logic.
• TaskInstance log reading behavior is unchanged (existing tests pass).
• Callback log reading (Add API endpoint and UI for deadline callback log visibility #66610) continues to work for both executor and triggerer paths.

References

• PR: Add API endpoint and UI for deadline callback log visibility #66610
• Review note: Add API endpoint and UI for deadline callback log visibility #66610 (comment)