Extract shared resolve_snap helper from duplicate plugin implementations

[Copilot]

_resolve_snap was duplicated verbatim across snap_preseed_plugin.py and uc_prepare_plugin.py, creating a divergence risk for snap reference handling.

Changes

• imagecraft/plugins/_utils.py (new) — module-level resolve_snap(snap: str) -> str function; strips whitespace and converts name/channel → name=channel for snap prepare-image
• snap_preseed_plugin.py / uc_prepare_plugin.py — remove _resolve_snap instance methods; import and call resolve_snap from ._utils
• tests/unit/plugins/test_utils.py (new) — parametrized tests for resolve_snap covering plain names, whitespace, local .snap paths, and channel references

# Before: each plugin had its own private method
def _resolve_snap(self, snap: str) -> str:
    snap = snap.strip()
    if "/" in snap and not snap.endswith(".snap"):
        name, channel = snap.split("/", 1)
        snap = f"{name}={channel}"
    return snap

# After: shared helper imported in both plugins
from ._utils import resolve_snap