Better vg describe

[jltsiren]

Changelog Entry

To be copied to the draft changelog by merger:

• vg describe works better with old obsolete files.

Description

vg describe will try to continue reading an identified file even when header validation fails. It also knows which versions of GBWT / GBWTGraph structures should have tags.

[adamnovak]

@glennhickey @jltsiren It looks like maybe combining this with #4852 in the merge commit 2069097 somehow caused some failures in the vg deconstruct tests only in the Mac tests, and only in CI (I can't replicate it locally).

https://github.com/vgteam/vg/actions/runs/23182304807/job/67357696590#step:7:6214

not ok 32 - all nested sites found in double-nested SNP
#          got: '2'
#     expected: '3'
not ok 33 - level 2 site found in double-nested SNP
#          got: '0'
#     expected: '1'

(I think we're also missing a bcftools dependency for the Mac tests, but that isn't causing this problem.)

[adamnovak]

Looking at the code, that doesn't make very much sense.