tagMetadata custom property without x report error on wrong location

[timotheeguerin]

should be on custom

import "@typespec/openapi";

using OpenAPI;

@service
@tagMetadata("ab", #{ custom: "Abc"})
namespace Test;

[Playground Link](https://typespec.io/playground/?e=%40typespec%2Fopenapi3&c=aW1wb3J0ICJAdHlwZXNwZWMvb3BlbmFwaSI7Cgp1c2luZyBPcGVuQVBJOwoKQHNlcnZpY2UKQHRhZ01ldGFkYXRhKCJhYiIsICN7IGN1c3RvbTogIkFiYyIgfSkKbmFtZXNwYWNlIFRlc3Q7Cg%3D%3D&options=%7B%7D&vs=%7B%7D

[github-actions]

🎯 Issue Triage

When using @tagMetadata with a custom property value that doesn't have an x- prefix (e.g., custom: "Abc"), an incorrect "wrong location" error is reported — the error should instead flag the missing x- prefix on the custom property key.

Category: Bug
Area: lib:openapi

🔗 [Playground Reproduction]((typespec.io/redacted)

📋 Reproduction code

import "`@typespec/openapi`";

using OpenAPI;

`@service`
`@tagMetadata`("ab", #{ custom: "Abc"})
namespace Test;

🔍 Analysis

• The @tagMetadata decorator in @typespec/openapi supports a custom property for adding extension fields to tags in the OpenAPI output.
• Extension fields in OpenAPI must be prefixed with x-. When a custom property key lacks this prefix, the error reported appears to be a "wrong location" diagnostic instead of a more accurate "missing x- prefix" error.
• The bug is likely in how the @tagMetadata decorator validates the custom record keys — the validation logic may be using the wrong diagnostic code or checking conditions in the wrong order.
• Affected package: @typespec/openapi

📝 Suggested next steps

• Locate the @tagMetadata decorator implementation in packages/openapi/src/
• Fix the validation to emit the correct diagnostic (missing x- prefix) instead of "wrong location"
• Add a test case covering custom properties without the x- prefix

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