[NopeCharon]

Fixes #494

ExpandRawFile refused an expansion unless the pool's remaining capacity could hold the volume's whole new size. get_capacity() already excludes the physical size of every volume in the pool, including the one being expanded, so the volume's existing blocks were counted twice. Once a volume was larger than the space left in its pool, it couldn't be expanded at all, even by a small amount.

Changes:

• ExpandRawFile now compares the capacity to what the expansion really takes:
  • thin volumes: the size increase, as truncate only grows the file.
  • thick volumes: new_size minus the image's physical size, as fallocate also allocates any holes (for example in an image restored from a sparse snapshot or discarded by mkfs). Using only the size increase would let such an expansion eat into the reserved capacity.
• tests/unit/test_internal_svc.py covers both provisioning modes, an increase that doesn't fit, and a thick image with holes.
• docs/user-guide/resize.md and CHANGELOG.md are updated.

Testing:

• .ci/unit.sh-equivalent run: 26 passed, 1 skipped. On macOS, so I ran pytest directly with the Linux-only _fallocate extension stubbed. CI will be the first run with the real one.
• With only the old condition restored, the two "larger than remaining capacity" tests fail. With the size increase used for thick volumes too, the holes test fails.
• ruff check / ruff format --check (v0.17.3, as in pre-commit) are clean.
• I called ExpandRawFile against a real pool directory with real statvfs, get_capacity and metadata, stubbing only losetup and fallocate. For an 8MiB volume with 4MiB left, expanded to 10MiB:
  • develop: RESOURCE_EXHAUSTED, for both thin and thick.
  • This change: OK, and the image and metadata are 10MiB. With 1MiB left, it is still RESOURCE_EXHAUSTED and the volume is unchanged.

I haven't run this on a cluster, so the smoke and e2e suites are left to CI.

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