Fix: Don't report non-literal-string-template when interpolating ErrorType

[Copilot]

• Understand the issue: When interpolating an invalid reference in a string template, both invalid-ref and non-literal-string-template errors are reported
• Locate the code that reports non-literal-string-template error (in string-template-utils.ts)
• Add check to skip reporting non-literal-string-template when the type is ErrorType
• Add test case to verify the fix works
• Run existing tests to ensure no regressions (all 3564 tests passed)
• Verify the fix manually (confirmed only invalid-ref is shown for invalid references, and non-literal-string-template still appears for valid non-serializable types)
• Run format and lint (all passed)
• Add changeset for the fix
• Address PR review feedback: Use isErrorType helper function instead of manual check

Original prompt

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This section details on the original issue you should resolve

<issue_title>Interpolating an invalid reference include confusing error</issue_title>
<issue_description>```tsp
const b = "Some ${bad}";

invalid-ref Unknown identifier bad
non-literal-string-template Value interpolated in this string template cannot be converted to a string. Only literal types can be automatically interpolated.

[Playground Link](https://typespec.io/playground/?e=%40typespec%2Fopenapi3&options=%7B%7D&vs=%7B%7D&version=1.4.x&c=Y29uc3QgYiA9ICJTb21lICR7YmFkfSI7Cg%3D%3D)

`non-literal-string-template` should probably not be reported if the interpolated type is already an errorType</issue_description>

<agent_instructions>When reporting non-literal-string-template we should ignore ErrorType</agent_instructions>

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

<comments>
</comments>

• Fixes Interpolating an invalid reference include confusing error #9606

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commit: 2dc08be

[github-actions]

All changed packages have been documented.

• ✅ @typespec/compiler

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@typespec/compiler - fix ✏️

Don't report non-literal-string-template diagnostic when interpolating an invalid reference

[azure-sdk]

You can try these changes here

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