[TypeSpec CI] Enable "--warn-as-error"

[mikeharder]

• Fixes [TypeSpec CI] Add "--warn-as-error" to "tsp compile" #24341

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️️✔️BreakingChange succeeded [Detail] [Expand]

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️️✔️Breaking Change(Cross-Version) succeeded [Detail] [Expand]

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Validation passes for ServiceAPIReadinessTest.

️️✔️SwaggerAPIView succeeded [Detail] [Expand]

️️✔️CadlAPIView succeeded [Detail] [Expand]

️️✔️TypeSpecAPIView succeeded [Detail] [Expand]

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️️✔️CadlValidation succeeded [Detail] [Expand]

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️️✔️TypeSpec Validation succeeded [Detail] [Expand]

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️️✔️PR Summary succeeded [Detail] [Expand]

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