Fix handling of DU members that are also DUs

[jhendrixMSFT]

Avoid spurious import statement for DU types when in unions.rs. Added Spector coverage for the impacted scenario.

[Copilot]

Pull request overview

This PR fixes Rust emitter support for discriminated unions that contain other discriminated unions, and adds a new Spector test crate (“hierarchy-building”) to validate the behavior end-to-end.

Changes:

• Update TCGC adapter + codemodel types to allow discriminated-union members to be either a Model or another DiscriminatedUnion.
• Update union codegen to avoid generating invalid/self-conflicting use statements when a DU contains another DU.
• Add new Spector test crate + integration tests for hierarchy-building, and bump @azure-tools/typespec-rust to 0.37.1 with changelog entry.

Reviewed changes

Copilot reviewed 10 out of 24 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/typespec-rust/src/tcgcadapter/adapter.ts	Emits DU members as nested DUs when subtypes are themselves polymorphic.
packages/typespec-rust/src/codemodel/types.ts	Broadens DU member type to DiscriminatedUnion | Model.
packages/typespec-rust/src/codegen/unions.ts	Skips adding use for DU members to prevent self-import conflicts in unions.rs.
packages/typespec-rust/package.json	Version bump to 0.37.1.
packages/typespec-rust/CHANGELOG.md	Adds 0.37.1 (unreleased) entry describing the DU-nesting fix.
packages/typespec-rust/.scripts/tspcompile.js	Registers new Spector input spector_hierarchy_building.
packages/typespec-rust/test/Cargo.toml	Adds hierarchy-building crate to the test workspace.
packages/typespec-rust/test/Cargo.lock	Adds lock entry for spector_hierarchy_building.
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/Cargo.toml	New Spector crate manifest for hierarchy-building scenario.
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/lib.rs	New crate entrypoint re-exporting generated code.
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/generated/mod.rs	Generated module root for clients/models.
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/generated/clients/mod.rs	Generated clients module exports.
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/generated/clients/hierarchy_building_client.rs	Generated top-level client + subclient accessors.
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/generated/clients/hierarchy_building_animal_operations_client.rs	Generated Animal operations client (PUT endpoints).
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/generated/clients/hierarchy_building_dog_operations_client.rs	Generated Dog operations client (PUT endpoint).
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/generated/clients/hierarchy_building_pet_operations_client.rs	Generated Pet operations client (PUT endpoints).
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/generated/models/mod.rs	Generated models module wiring.
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/generated/models/method_options.rs	Generated method option structs for hierarchy-building operations.
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/generated/models/models.rs	Generated Dog model.
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/generated/models/models_impl.rs	Generated conversions + request-content helpers for models.
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/generated/models/unions.rs	Generated discriminated unions (Animal, Pet) including nested DU membership.
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/generated/models/unions_impl.rs	Generated request-content helpers for unions.
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/src/generated/models/unions_serde.rs	Generated custom Serialize for extensible polymorphic unions.
packages/typespec-rust/test/spector/azure/client-generator-core/hierarchy-building/tests/hierarchy_building_tests.rs	New integration tests covering DU nesting + request/response roundtrips.

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