Handle short image reads

[simsong-codex]

Codex-authored fix for #507.

Closes #507

What changed

• Treat a positive raw-image short read as the actual buffer length rather than scanning the unread allocation tail.
• Apply the same safe sbuf construction to EWF short reads and EOF.
• Preserve the logical page size when a page plus margin is shortened.

Validation

• make -s -j4 -C src check TESTS=test_be
• The Linux test truncates an already-open raw image and verifies the reader exposes only the actual bytes. APFS retains the original view on an open descriptor, so that platform cannot reproduce this real short-read condition.

[codecov]

Codecov Report

❌ Patch coverage is 97.01493% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.79%. Comparing base (5abbd86) to head (93fc1cc).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/image_process.cpp	91.67%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #518      +/-   ##
==========================================
+ Coverage   73.41%   73.79%   +0.38%     
==========================================
  Files         113      113              
  Lines       12586    12643      +57     
==========================================
+ Hits         9239     9329      +90     
+ Misses       3347     3314      -33     

Flag	Coverage Δ
unittests	73.79% <97.01%> (+0.38%)	⬆️

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[Copilot]

Pull request overview

This pull request addresses issue #507 by making raw and EWF image readers handle positive short reads safely, ensuring scanners only see initialized bytes and that page-size semantics remain consistent when reads are truncated (e.g., due to EOF or live truncation).

Changes:

• Introduces safe “shrink-to-actual-bytes-read” behavior for sbuf_t construction when a read returns fewer bytes than requested (raw + EWF).
• Updates raw reader logic to use gcount() and treat positive short reads as valid buffers (and not scan unread allocation tail).
• Adds regression tests for real-world raw truncation behavior on Linux and for EWF short-read/EOF behavior (when libewf is available).

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/image_process.cpp	Implements short-read-safe sbuf_t construction for EWF/raw readers and correct raw short-read handling via gcount().
src/test_be3.cpp	Adds tests covering raw truncation short reads (Linux) and EWF short-read/EOF behavior (HAVE_LIBEWF).

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