feat(oci): allow host-less repos complemented by the KPM_REG envvar

[cjuega]

1. Does this PR affect any open issues?(Y/N) and add issue references (e.g. "fix #123", "re #123".):

• N
• Y (Support host-less OCI dependencies resolved via KPM_REG kpm#739)

2. What is the scope of this PR (e.g. component or file name):

cmd/kcl/commands/args.go, cmd/kcl/commands/mod_add.go, cmd/kcl/commands/args_test.go

3. Provide a description of the PR(e.g. more details, effects, motivations or doc link):

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Other

Allow --oci to accept a bare repository path with no scheme and no host
(e.g. myorg/kcl-templates/utils). When url.Parse returns an empty scheme
the argument is now treated as a host-less reference and an Oci source is
built with RegFromEnv: true, so KPM resolves the registry at runtime from the
KPM_REG environment variable or DefaultOciRegistry.

Previously, the same input would fall into the scheme-rewriting branch, which
prepended oci:// to an already schemeless string and produced a malformed URL.

This is useful for multi-account or air-gapped deployments where the same
kcl.mod must work against different registries without being modified:

export KPM_REG=123456789012.dkr.ecr.eu-west-1.amazonaws.com
kcl mod add --oci myorg/kcl-templates/utils --tag 0.2.0

Full OCI URLs (oci://ghcr.io/..., https://ghcr.io/...) are unaffected.

4. Are there any breaking changes?(Y/N) and describe the breaking changes(e.g. more details, motivations or doc link):

• N
• Y

The existing behaviour for fully-qualified OCI URLs is preserved unchanged.
The new code path is only reached when url.Parse returns an empty scheme,
which previously produced a malformed URL and would have errored downstream.

5. Are there test cases for these changes?(Y/N) select and add more details, references or doc links:

• Unit test
• Integration test
• Benchmark (add benchmark stats below)
• Manual test (add detailed scripts or steps below)
• Other

Four unit tests added in cmd/kcl/commands/args_test.go:

Test	What it covers
TestParseSourceFromArgs_HostlessOci	Bare path → RegFromEnv=true, registry empty
TestParseSourceFromArgs_HostnameWithoutScheme	ghcr.io/… without scheme → treated as bare path (Go's url.Parse cannot detect a host without ://); documents the edge-case behaviour explicitly
TestParseSourceFromArgs_FullOciUrl	Full oci://host/repo URL → Reg populated, RegFromEnv=false (regression guard)
TestParseSourceFromArgs_HostlessOciWithModSpec	Positional name:version + host-less --oci → both Oci and ModSpec fields populated correctly

[cjuega]

Be aware we need the changes in this PR before merging this. So we still have to bump kpm once the aforementioned change is released