fix(rpm-repos): address PR review feedback and lint failures

[Copilot]

Resolves Copilot review feedback on PR microsoft#156 and the failing Lint and Scenario tests checks (the latter caused by an out-of-date checked-in JSON schema snapshot).

Behavior changes

• validateRemoteURI: rejects opaque URIs (e.g. https:example.com) and requires a non-empty Host.
• validateGPGKey: same opaque rejection; http(s) requires Host, file:// requires empty Host + absolute Path (i.e. real file:///abs/path form).
• addConfiguredImageBuildRepos: no-op (info log) when inputs.image-build is unconfigured, instead of hard-failing. The .kiwi description and --repo flags continue to supply repos for those projects. The "all entries filtered out by arches" path is unchanged.

Schema

• RpmRepoResource.JSONSchemaExtend now mirrors the runtime constraints: oneOf(base-uri, metalink) and a tightened gpg-key pattern (^((https?|file)://\S+|[^\s:]\S*)$).
• Escaped commas in the gpg-key description struct tag — without this, the jsonschema reflector truncates the description at the first comma, which is why the previous schema only listed http.
• Regenerated schemas/azldev.schema.json (this is what was making TestCheckedInSchemasAreUpToDate fail).

Lint

• Renamed prepare_test.go and resources_test.go to *_internal_test.go (they exercise unexported symbols; the suffix is the repo's existing escape hatch from testpackage).
• //nolint:recvcheck on ResourcesConfig and RpmRepoResource — JSONSchemaExtend must be a value receiver because invopop/jsonschema only invokes value-receiver methods via t.Implements.
• Split validateRpmRepo into validateRpmRepoSource / validateRpmRepoGPG to drop cyclomatic complexity below 15 and remove the single-case switch.
• Extracted siteDefaultsPerm = 0o644 and schemeHTTP/HTTPS/File constants; renamed short variables; applied staticcheck QF1001/QF1002; reworded ST1005 errors; assert.Empty over assert.Equal(t, "", …).

Tests

• Added TestValidateRpmRepo cases for the new URI shape rules: opaque base-uri/metalink/gpg-key, missing host on https:///path, and file://server/share/....