Add unit tests for snap-preseed-channel and snap-preseed-validation plugin options

[Copilot]

SnapPreseedPluginProperties exposes snap-preseed-channel and snap-preseed-validation, but neither had test coverage verifying the CLI flags were correctly wired into the generated snap prepare-image command.

Changes

• test_get_build_commands_with_channel: asserts --channel=<value> appears in the command when snap-preseed-channel is set
• test_get_build_commands_with_validation_enforce: asserts --validation=enforce replaces the default --validation=ignore when snap-preseed-validation: enforce is specified

# Channel flag wired correctly
properties = SnapPreseedPluginProperties.unmarshal({
    "snap-preseed-snaps": ["core24"],
    "snap-preseed-channel": "latest/stable",
})
# → snap prepare-image --classic --arch=amd64 --validation=ignore --channel=latest/stable --snap=core24 "" <install_dir>

# Validation override wired correctly
properties = SnapPreseedPluginProperties.unmarshal({
    "snap-preseed-snaps": ["core24"],
    "snap-preseed-validation": "enforce",
})
# → snap prepare-image --classic --arch=amd64 --validation=enforce --snap=core24 "" <install_dir>