fix: handle missing DiffIDs in tarball Manifest() to prevent panic

[Yanhu007]

Fixes #2192

Problem

(*compressedImage).Manifest() accesses cfg.RootFS.DiffIDs[i] without bounds checking. When loading a tar image whose config is a non-standard type (e.g. application/vnd.buildkit.cacheconfig.v0), DiffIDs is empty, causing:

panic: runtime error: index out of range [0] with length 0

Fix

Add a bounds check before accessing DiffIDs[i]. When the index exceeds the DiffIDs length (as with buildkit cache configs), fall through to read the layer directly from the tar archive to compute its digest and size, similar to the existing non-foreign-layer path.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 52.86%. Comparing base (8b3c303) to head (e860ebb).
⚠️ Report is 90 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/v1/tarball/image.go	0.00%	13 Missing and 1 partial ⚠️

❗ There is a different number of reports uploaded between BASE (8b3c303) and HEAD (e860ebb). Click for more details.

HEAD has 1 upload less than BASE

Flag	BASE (8b3c303)	HEAD (e860ebb)
	2	1

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2263       +/-   ##
===========================================
- Coverage   71.67%   52.86%   -18.82%     
===========================================
  Files         123      165       +42     
  Lines        9935    11221     +1286     
===========================================
- Hits         7121     5932     -1189     
- Misses       2115     4574     +2459     
- Partials      699      715       +16     

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[Subserial]

Hey @Yanhu007, you'll need to sign the CLA before we can accept this PR.