manage: extract _fetch_image_info to reduce duplication

[ideaship]

ImageClusterapi, ImageClusterapiGardener, and ImageOctavia all share
the same marker-fetch and checksum-fetch sequence: fetch a marker
file, parse the date and image filename, construct the image URL,
fetch the .CHECKSUM file, and log each step. This identical block
was repeated verbatim in all three take_action() implementations.

Extract this sequence into a private _fetch_image_info(base_url,
marker_url) helper that returns (date, image_filename, url, checksum).
Callers that need the image filename for version extraction
(ImageClusterapi, ImageClusterapiGardener) unpack it; ImageOctavia
discards it with _.

ImageGardenlinux is deliberately excluded: it constructs the image
URL directly from a known pattern rather than fetching a marker file,
so it shares only the checksum-fetch half of the pattern and does not
fit this helper without contortion.

AI-assisted: Claude Code
Signed-off-by: Roger Luethi luethi@osism.tech

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[sourcery-ai]

Hey - I've left some high level feedback:

• In _fetch_image_info, consider validating the number of whitespace-separated fields in the marker body before doing strip().split()[:2] so that a malformed marker produces a clear, custom error instead of an unhandled ValueError from tuple unpacking.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- In `_fetch_image_info`, consider validating the number of whitespace-separated fields in the marker body before doing `strip().split()[:2]` so that a malformed marker produces a clear, custom error instead of an unhandled `ValueError` from tuple unpacking.

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