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

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…