fix: support multi-layer inheritance in hierarchyBuilding decorator

[Copilot]

The @hierarchyBuilding decorator was failing validation when skipping multiple inheritance layers. It only checked direct properties on the target model, not inherited properties.

@Azure.ClientGenerator.Core.Legacy.hierarchyBuilding(C)
model A extends B { a?: string; }
model B extends C { b?: string; }
model C { c?: string; }

Previously this would report a warning that A is missing property c and the decorator would not apply.

Changes

• decorators.ts: Added getAllPropertiesInChain() to collect all properties from a model's inheritance chain. Updated isPropertySuperset() to validate against inherited properties, not just direct properties.

• types.ts: When legacy hierarchy building skips intermediate models, properties from those skipped models are now "collapsed" into the target SDK type.

Result

Model A now correctly:

• Has base type C
• Contains properties a (own) and b (from skipped model B)
• Model C has property c

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• telemetry.astro.build
  • Triggering command: /usr/local/bin/node node /home/REDACTED/work/typespec-azure/typespec-azure/website/node_modules/.bin/../astro/astro.js build node ical/node_modules/.bin/../@typespec/tspd/cmd/tspd.js tput-dir ../../website/src/content/docs/docs/emitters/protobuf/reference . .0_tmp_3782/node_modules/pnpm/dist/node-gyp-bin/sh ern-signature &&sh ce n/sh sh c-az�� tput-dir ../../website/src/conte--llmstxt node n/sh perimental gen-esh gen-extern-signa-c ager/reference node (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[TCGC] improve the hierarchyBuilding decorator to support multiple layer inheritance replacement</issue_title>
<issue_description>Now the decorator implementation of this hierarchyBuilding decorator is checking properties very strictly.

For instance, in the following case, the decorator will report warning and would not apply.

@Azure.ClientGenerator.Core.Legacy.hierarchyBuilding(C)
model A extends B {
a?: string;
}
model B extends C {
b?: string;
}
model C {
c?: string;
}

We need to implement this so that in the output of TCGC, we have model A with base type of C, and A should have property a (defined on its own) and b (from model B) on it, model C has property c.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes [TCGC] improve the hierarchyBuilding decorator to support multiple layer inheritance replacement #3737

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