Fix bug: Unable to use {service-name} interpolation for filenames with openapi3-emitter when only one service is defined

[wanlwanl]

Fix: #5866

[azure-sdk]

All changed packages have been documented.

• ✅ @typespec/openapi3

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@typespec/openapi3 - breaking ✏️

Using {service-name} in tspconfig.yaml will always interpolate the current service name. {service-name-if-multiple} can be used to get the previous behavior

PR Overview

This PR fixes the bug where using the {service-name} interpolation for filenames with the openapi3-emitter fails when only one service is defined. The changes split the interpolation into two variables ("service-name" and "service-name-if-multiple") and update the output file patterns, documentation, and tests accordingly.

• Created a changelog file documenting the fix.
• Updated emitter configuration and interpolation logic in the source code.
• Modified tests and documentation to reflect the new interpolation behavior.

Reviewed Changes

File	Description
.chronus/changes/wanl-service-name-2025-1-27-19-40-32.md	Changelog entry for the bug fix.
packages/openapi3/test/test-host.ts	Updated YAML parsing import and introduced new EmitterOptions usage.
packages/openapi3/src/openapi.ts	Adjusted output file interpolation with separate keys for service names.
packages/openapi3/test/works-for.ts	Extended test helpers to include new emitOpenApi API and interpolation.
website/src/content/docs/docs/emitters/openapi3/reference/emitter.md	Revised documentation for the output file interpolation keys.
packages/openapi3/test/emit-openapi.test.ts	Added tests to validate expected file naming with single and multiple services.
packages/openapi3/src/lib.ts	Updated emitter option schema and documentation to reflect new defaults.

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

Comments suppressed due to low confidence (2)

packages/openapi3/src/openapi.ts:203

• Ensure that the change in the default output file template using {service-name-if-multiple} is intentional and clearly documented so that users understand the intended substitution behavior for single versus multiple services.

resolvedOptions["output-file"] ?? `openapi.{service-name-if-multiple}.{version}.${fileType}`;

packages/openapi3/test/emit-openapi.test.ts:109

• For the test case using the {service-name-if-multiple} pattern with one service, the expected output becomes just 'yaml', which might indicate that no service name is interpolated. Please verify that this behavior is intentional and consistent with the overall design.

expectedOutputFiles: [resolveVirtualPath("yaml")],

[azure-sdk]

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