Add @nullFinalStateVia legacy decorator for LRO backward compatibility

[Copilot]

Context:

• Support to omit final-state-via in LRO operations #3648
• teams discussion

Summary:

Adds @nullFinalStateVia decorator to force LRO operations to have finalStateVia set to undefined, supporting backward compatibility for language clients that require this specific behavior.

Changes

• Decorator definition (lib/legacy.tsp): Added @nullFinalStateVia(target: Operation, scope?: valueof string) with scope support
• Type system (generated-defs/, src/decorators.ts, src/tsp-index.ts): Added NullFinalStateViaDecorator type and $nullFinalStateVia handler
• Helper function (src/decorators.ts): Exported getNullFinalStateVia(context, operation): boolean for language emitters
• Interface update (src/interfaces.ts): Changed SdkLroServiceMetadata.finalStateVia from required to optional
• Logic (src/methods.ts): Modified getServiceMethodLroMetadata() to return undefined for finalStateVia when decorator is applied
• Tests (test/decorators/null-final-state-via.test.ts): Added coverage for scoping, negation, and interaction with @markAsLro and @finalStateVia

Usage

// Apply to all emitters
@Azure.ClientGenerator.Core.Legacy.nullFinalStateVia
@Azure.Core.lroPollLocation
op startJob(@path jobId: string): JobResult;

// Language-specific
@Azure.ClientGenerator.Core.Legacy.nullFinalStateVia("python")
@Azure.Core.lroPollLocation
op processData(): ProcessingResponse;

Language emitters check via getNullFinalStateVia(context, operation) or test metadata.finalStateVia === undefined.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[TCGC] add a legacy decorator @nullFinalStateVia</issue_title>
<issue_description>Goal:

add a legacy decorator @nullFinalStateVia to backward compatibility for some language clients

Ask:
(1) this decorator shall be put in /workspaces/typespec-azure/packages/typespec-client-generator-core/lib/legacy.tsp
(2) this decorator shall pertmit "scope"
(3) add helper function for a method so that language emitter could judge the method is decorated by this decorator
(4)

typespec-azure/packages/typespec-client-generator-core/src/interfaces.ts

Line 1021 in a68aa69

finalStateVia: FinalStateValue;

shall be changed to optional
(5) finalStateValue shall be undefined if a lro method is decorated by it
(6) DO add test case
(7) DO add changelog in .chronus/changes

</issue_description>

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

• Fixes [TCGC] add a legacy decorator @nullFinalStateVia #3707

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commit: 936a44c

[timotheeguerin]

Catching up on this, was this completely agreed upon?
I really don't like this name. It is too descriptive of how you defined that in swagger and not of the behavior we are expecting here.