osd/ECUtil: Fix erase_after_ro_offset length calculation and add tests

[aainscow]

System test logs showed EC recovery failures with assertion errors when
recovering small objects (smaller than stripe width) in EC pools.
The recovery would fail with "shard_size >= tobj_size" assertions
because shards that should be empty incorrectly contained data.

The primary change in this commit fixes a bug in
shard_extent_map_t::erase_after_ro_offset() where the length
calculation was incorrect:

sinfo->ro_range_to_shard_extent_set(ro_offset, ro_end - ro_start, ...)

Should be:

sinfo->ro_range_to_shard_extent_set(ro_offset, ro_end - ro_offset, ...)

When ro_offset < ro_start, the incorrect calculation caused data that
should be erased to remain on shards, leading to recovery failures.

Additionally, this commit adds 13 comprehensive unit tests to TestECUtil
that thoroughly exercise erase_after_ro_offset across various edge cases,
including the critical scenario of objects smaller than stripe width where
some shards should remain empty. These tests successfully catch the bug
when it is re-introduced.

Note: The unit tests in this commit were generated with assistance from
an LLM (Large Language Model) and subsequently validated and refined.

Fixes: https://tracker.ceph.com/issues/74329

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[bill-scales]

LGTM

[rzarzynski]

The fix LGTM!

The comments are just about the primary unit test which, per the on-Slack discussion, needs some changes.

[ljflores]

jenkins test make check arm64

[ljflores]

jenkins test windows

[ljflores]

RADOS approved: https://tracker.ceph.com/projects/rados/wiki/MAIN#httpstrackercephcomissues74521

[aainscow]

jenkins test make check