Add deterministic default+custom toolset resolution regression

[davidahmann]

Problem

When default and explicit toolset selections overlap, ordering and deduplication behavior must remain deterministic to keep exposed toolsets and resulting inventories stable.

Why now

Issue #2104 tracks deterministic toolset resolution ordering for default/custom combinations.

What changed

• Added TestWithToolsets_DefaultAndCustomOrderIsDeterministic in pkg/inventory/registry_test.go.
• The test asserts deterministic enabled toolset order and deterministic available tool ordering for overlapping default + explicit toolset inputs.

Validation

• go test ./pkg/inventory -run 'TestWithToolsets_DefaultAndCustomOrderIsDeterministic|TestWithToolsets'

Refs #2104

[davidahmann]

Implemented issue #2104 with additional deterministic toolset-resolution coverage.

What changed:

• Added TestWithToolsets_DefaultAndCustomOrderIsDeterministic in pkg/inventory/registry_test.go.
• The test asserts stable enabled toolset ordering and stable tool ordering when default and explicit toolsets overlap.

Validation:

• go test ./pkg/inventory -run 'TestWithToolsets_DefaultAndCustomOrderIsDeterministic|TestWithToolsets' ✅

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