Motivation

• JPEG marker-scanning logic was duplicated across _get_size, _get_dpi, and _get_channels, making the code harder to maintain and reason about.
• Consolidate JPEG segment parsing to reduce duplication and centralize error handling for robustness.

Description

• Introduced _JPEG_NO_SOF_MARKERS to list JPEG markers that are not SOF markers and used it in SOF detection.
• Added helper def _read_jpeg_segment_header(fhandle) to read a JPEG segment header and validate its size.
• Added helper def _seek_to_jpeg_sof(fhandle) to seek to the next SOF marker, replacing inline marker-scanning loops.
• Replaced duplicated loops in _get_size, _get_dpi, and _get_channels to call the new helpers and unified JPEG-related exception handling to raise ValueError on parse errors.

Testing

• Ran the full test suite with python -m pytest -q and all tests passed (82 passed).
• No regressions observed in image parsing behavior according to the existing tests.

Codex Task