generator: generated methods return a promise without .cancel() since request/response logging was added #9790

Library Name: all generated clients, for example @google-cloud/kms (generator: core/generator/gapic-generator-typescript)

Try this API: Not applicable. The problem is the promise the client returns, not the API response.

Repro link: https://gist.github.com/aelmanaa/09946f41f4779c59c4d324e40d6827d2

Steps:

1. Start a local HTTP server that waits 2 s before it answers.
2. Create KeyManagementServiceClient({fallback: true, protocol: 'http', apiEndpoint: '127.0.0.1', port, authClient: new PassThroughClient()}).
3. Run const p = client.asymmetricSign(request) and check typeof p.cancel.
4. After 200 ms, call p.cancel() if it exists.

Description:
client-libraries.md in gax says a method's returned promise can be cancelled with promise.cancel(). Generated clients no longer return that promise. To log the response, each method now returns this.innerApiCalls.X(request, options, wrappedCallback)?.then(...). .then() returns a plain Promise, so the OngoingCallPromise from gax and its cancel() are lost.

Results with the repro (Node 24, REST fallback):

• @google-cloud/kms 4.5.0 (gax 4.6.1): typeof p.cancel is function. cancel() closes the connection immediately.
• @google-cloud/kms 5.0.0 (gax 5.0.8) and packages/google-cloud-kms on main (0435467, gax 6.14.0): typeof p.cancel is undefined. The request runs until the server answers.
• On main, client.innerApiCalls.asymmetricSign(request, {}) still returns a promise with cancel(), and it aborts the fetch.

This came from the logging template change in googleapis/gapic-generator-typescript#1671. The ?.then(...) wrapper is in templates/{cjs,esm}/.../src/$version/$service_client.ts.njk for normal, LRO, diregapic LRO and paged methods.

#9462 added CallOptions.signal, not released yet. With gax main, client.asymmetricSign(request, {signal}) aborts the request because options still reaches gax. So a way to cancel is coming, but the documented one is broken.

Expected: one of:

• The returned promise keeps cancel(), for example by copying cancel from the inner promise onto the .then() result and typing the return as gax.CancellablePromise<...>.
• client-libraries.md drops promise.cancel() and documents options.signal once gax 6.15.0 is out.

I can send a PR for either.

Why: core/packages/gax/client-libraries.md ("Regular methods") documents promise.cancel(), and it worked before the logging change.

Versions: google-cloud-node main 0435467, @google-cloud/kms 6.3.0, google-gax 6.14.0, Node 24.21.0.