Use JSON Schema defaults in synthetic test get_diff

[Steve Lee (SteveL-MSFT)]

Motivation

When a DSC resource does not implement the test operation, the engine performs a synthetic test by calling get and comparing expected vs actual state using get_diff(). Previously, if a property existed in the expected (desired) state but was missing from the actual (get) output, it was always reported as differing -- even if the resource simply omits properties whose values match the schema-defined default.

This causes false positives in synthetic test results. The most impactful case is Microsoft.Windows/FirewallRuleList where unspecifiedRulesAction is a write-only instruction that get never returns. Setting it to "ignore" (the documented default) permanently reports drift, making the property unusable for compliance reporting.

Approach

• Introduced get_diff_with_schema(expected, actual, schema) which accepts an optional JSON Schema. When a property is in expected but absent from actual, the function looks up the schema's default value for that property. If the expected value matches the default, it is not flagged as a diff.
• The existing get_diff(expected, actual) remains as a convenience wrapper (passes None for the schema), so all non-synthetic-test call sites are unaffected.
• Both synthetic test paths (command resources and adapted resources) now retrieve the resource schema and pass it to the new function.
• Schema is read directly from the RESOURCE_SCHEMAS cache (avoiding redundant serialization), with a fallback to get_schema() to populate the cache on a miss.

Test coverage

• Rust unit tests (6 new): Cover matching/mismatching schema defaults for booleans and integers, missing defaults, and the no-schema fallback.
• Pester integration tests (7 new): End-to-end validation using a new Test/SchemaDefault resource.
• FirewallRuleList Pester tests (4 new): Verify that unspecifiedRulesAction: ignore no longer causes false drift, while non-default values (disable, remove) are still correctly reported.
• Verified existing dsc_resource_test.tests.ps1 tests still pass with no regressions.

Fixes #1666

[Copilot]

Pull request overview

Adds schema-aware diffing for synthetic dsc resource test (when a resource lacks a native test operation), so missing properties in get output don’t produce false diffs when the desired value matches the JSON Schema default.

Changes:

• Introduces get_diff_with_schema(expected, actual, schema) and a helper to read per-property schema defaults when actual omits a key.
• Updates synthetic test paths (command + adapted resources) to retrieve the resource schema and pass it into diffing.
• Adds new Rust + Pester tests, including a Test/SchemaDefault resource that omits defaulted fields from get output.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
tools/dsctest/src/schema_default.rs	Adds a dsctest input struct for the new schema-default test resource.
tools/dsctest/src/main.rs	Adds schema-default subcommand and exposes its JSON schema via dsctest schema.
tools/dsctest/src/args.rs	Wires new clap subcommand and schema enum variant.
tools/dsctest/dsctest.dsc.manifests.json	Adds Test/SchemaDefault manifest with embedded schema defaults.
lib/dsc-lib/src/dscresources/dscresource.rs	Adds schema-aware diffing and unit tests for default handling.
lib/dsc-lib/src/dscresources/command_resource.rs	Passes resource schema into synthetic test diffing.
dsc/tests/dsc_schema_default.tests.ps1	Adds end-to-end tests verifying schema defaults are honored in synthetic tests.

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[github-actions]

😁 Code Coverage Report

Changed Code Coverage

90% (90%+ coverage)

Metric	Value
Changed lines analyzed	85
Lines covered by tests	77
Coverage percentage	90%

🔵 Full Codebase Coverage

82% (good)

Metric	Value
Total executable lines	18748
Lines covered by tests	15431
Coverage percentage	82%

Changed code coverage measures only Rust lines added/modified in this PR.
Full codebase coverage measures all instrumented Rust lines across the project.

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

Overall, this PR looks good to me.

Out of scope but related, we should probably leverage this new comparison with schemas in a follow up PR to handle write-only and read-only properties that differ between actual/expected - write-only properties can never be compared and there's an argument for read-only properties only being compared when they're in both states (because the user is specifically saying "I expect output to be foo").

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

Nit: Out of scope for this PR, but I think this fits better as an extension method in dsc-lib-jsonschema - probably something like the following signature:

pub get_default_property_value(
  &Self,
  property_name: &str,
) -> Option<&Value> {}

[Steve Lee (SteveL-MSFT)]

Agree we can improve this outside of this PR