OpenAPI emitter: expose setOperationId function to set and modify operations in other emitters

[Copilot]

Fixes #8511 by exposing a public setOperationId function that allows other emitters to programmatically set operation IDs on operations.

Problem

Previously, emitters could only read operation IDs using getOperationId() but had no way to programmatically set them. The only way to set operation IDs was through the @operationId decorator, which couldn't be applied programmatically from JavaScript/TypeScript code. This made it impossible for custom emitters to modify operation IDs before passing them to the OpenAPI emitter.

Solution

Added a new setOperationId(program: Program, entity: Operation, opId: string): void function that:

• Allows programmatic setting of operation IDs from any emitter
• Uses the same internal state management as the @operationId decorator
• Is properly exported from the OpenAPI library's public API
• Integrates seamlessly with existing getOperationId() and resolveOperationId() functions

Usage

import { setOperationId, getOperationId } from "@typespec/openapi";

// In your custom emitter:
function customEmitter(program: Program) {
  for (const [name, operation] of program.getGlobalNamespaceType().operations) {
    if (!getOperationId(program, operation)) {
      // Set custom operation ID based on your logic
      setOperationId(program, operation, `custom_${operation.name}`);
    }
  }
}

The OpenAPI emitter will automatically pick up these programmatically set operation IDs when generating specifications.

Testing

Added comprehensive test coverage including:

• Basic programmatic operation ID setting
• Interaction with decorator-set operation IDs
• Override scenarios in both directions
• Integration with existing getOperationId() functionality

All existing tests continue to pass, ensuring no breaking changes.

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Original prompt

---

This section details on the original issue you should resolve

<issue_title>OpenAPI emitter: expose setOperationId function to set and modify operations in other emitters</issue_title>
<issue_description>### Clear and concise description of the problem

I wanted to workaround this issue:

#8511

And figured that I can't that easily programmatically set operationId on an operation and pass modified program to openapi emitter.

I can't apply decorators from emitter (no js api for this).
And openapi emitter doesn't expose setOperationId function, only getOperationId.

So, I'd like to either have an openAPI exposed function to setOperationId on an operation, modifying the program.
Or have a way to programmatically set decorators using compiler API.

Checklist

• Follow our Code of Conduct
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.</issue_description>

<agent_instructions>Make the setOperationId helper function a public function exported from the OpenApi library</agent_instructions>

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

Fixes #8576

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