fix(gen-jsonschema): fix allOf $ref schema naming and sort closure bugs

[project0]

hint: This bug was solved with the help of AI (Claude)

I discovered this problem when generating schemas for cloud-init: https://github.com/canonical/cloud-init/blob/main/cloudinit/config/schemas/schema-cloud-config-v1.json

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Three bugs in the JSON schema to KCL generator caused schemas using the allOf + placeholder-properties pattern (e.g., cloud-init) to produce incorrect output:

1. Sort closure variable shadowing: sort.SliceStable closures used i,j parameter names that shadowed the outer loop variable, causing $ref (order 0) to sort after properties (order 2), so properties were converted before allOf refs could merge in real types.

2. Empty placeholder properties not replaced: when merging allOf member properties into an existing schema, properties with empty keyword sets (placeholder schemas like "count": {}) were silently skipped instead of being replaced with the typed schema from the allOf member.

3. $ref residue causing wrong schema names: the first $ref encountered in an allOf fell through to default: and was stored in s.Keywords rather than resolved, causing the main keyword loop to later set reference = "#/$defs/first_ref" and derive the schema name from it instead of from $id or the filename. Fixed by adding case *jsonschema.Ref: to the outer allOf switch so all refs are always resolved into schs immediately.

Adds a minimal regression test (allof-placeholder-props) and the previously missing cloud-init/expect.k test fixture.

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

• N (not sure)
• Y

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

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

• N
• Y

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• Unit test
• Integration test
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