fix(@typespec/openapi): Report invalid-extension-key error on the offending property name, not the decorator argument

[Copilot]

When using @tagMetadata with a custom property missing the x- prefix, the invalid-extension-key diagnostic was reported at the wrong location — pointing to the tag name argument instead of the invalid property.

@service
@tagMetadata("ab", #{ custom: "Abc"})  // error was on "ab", should be on `custom`
namespace Test;

Changes

• helpers.ts — checkNoAdditionalProperties now accepts an optional ObjectLiteralNode and resolves the specific ObjectLiteralPropertyNode by name, using its id identifier node as the diagnostic target. For nested model properties, the inner ObjectLiteralNode is threaded through recursive calls. validateAdditionalInfoModel gains a corresponding optional node parameter.

• decorators.ts — tagMetadataDecorator was incorrectly using getArgumentTarget(0) (the tag name string) for all tagMetadata validations; fixed to getArgumentTarget(1) and now passes the resolved ObjectLiteralNode for precise targeting. Same improvement applied to $info.

• Tests — Added two position-sensitive tests using extractSquiggles that assert the diagnostic lands exactly on the invalid property name identifier, both at the root level and within nested objects (e.g., externalDocs). Added PlainTester to the test host for position-sensitive tests that need full control over the compiled source.