[FEATURE REQ] Support .tsp-only PRs in incremental TypeSpec detection for ARM Auto SignOff

### API Spec link

N/A

### API Spec version

N/A

### Please describe the feature.

Hi @AkhilaIlla , @mikeharder , @raosuhas, could the incremental TypeSpec detection be extended to support PRs that only modify .tsp files?

Currently, arm-incremental-typespec.js maps changed files to spec directories using only `swagger()`, `example()` and `readme()` filters, so .tsp-only PRs always result in an empty `changedSpecDirs` and return false. This means TypeSpec-only changes to an existing TypeSpec RP can't receive ARMAutoSignedOff-IncrementalTSP and must wait for manual ARM review.

Adding .tsp files (via the existing `typespec()` filter) to the `changedSpecDirs` computation would address this, and the existing base-branch check (verifying TypeSpec-generated swagger already exists) would still guard against false positives.

This would significantly reduce review wait times for teams working in TypeSpec. We recently migrated from raw swagger to TypeSpec, and we're working on refactoring the tsp files to resolve some FIXMEs left by the auto conversion tool. These updates will not alter the generated swagger, ensuring we don't introduce any unintended changes. For instance, this is PR #40221, along with the [current outcome](https://github.com/Azure/azure-rest-api-specs/actions/runs/21810787352/job/62922229100?pr=40188) from 'Checking for incremental TypeSpec changes'.
> 2026-02-09T02:57:36.268Z simple-git [GitExecutor] [SPAWN] git [ 'diff', '--name-status', 'HEAD^', 'HEAD' ]
> Could not map changed files to any spec directories
> Incremental TypeSpec result: false

Thanks for considering!

cc @matthchr , @palma21