Fix nested diff w/ schema and schema cache miss

[Steve Lee (SteveL-MSFT)]

PR Summary

• Enhance when there is a schema cache miss
• Fix synthetic test diff when value is an object to also consider writeOnly properties
• Fix test schema to be consistent with the embedded schema

[Mikey Lombardi (He/Him) (michaeltlombardi)]

This looks pretty coherent to me. My only request would be a test to validate that it behaves as expected, particularly for the nested property with a reference.

[Copilot]

Pull request overview

Improves synthetic test diffing by making schema-aware comparisons work correctly for nested object properties (including referenced schemas and writeOnly handling) and by ensuring schema comparisons still work when the schema cache is cold/missed.

Changes:

• Pass the relevant nested schema into recursive diffing and resolve local $refs so nested writeOnly/default metadata is honored.
• On schema cache miss, parse the schema returned by get_schema and use it immediately (with best-effort cache population).
• Extend the SchemaDefault test resource/schema and add Pester coverage for nested writeOnly objects and $ref-referenced nested schemas.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
tools/dsctest/src/schema_default.rs	Extends the SchemaDefault test input model to include nested objects and defaults, aligned with camelCase JSON.
tools/dsctest/src/main.rs	Adjusts SchemaDefault synthetic “actual state” output to include nested objects when present in input.
tools/dsctest/dsctest.dsc.manifests.json	Updates the embedded schema for Test/SchemaDefault to include nested and referenced nested definitions with writeOnly secret handling.
lib/dsc-lib/src/dscresources/dscresource.rs	Enhances get_diff_with_schema to recurse with the correct nested schema and resolve local $refs for accurate nested comparisons.
lib/dsc-lib/src/dscresources/command_resource.rs	Improves synthetic test behavior on schema cache miss by parsing/using the returned schema directly and caching it best-effort.
dsc/tests/dsc_schema_default.tests.ps1	Adds tests verifying nested writeOnly objects are ignored and non-writeOnly nested differences are reported (including $ref cases).

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