fix(dart_pub_publish): requires pub_credentials secret despite using OIDC #540

Description

dart_pub_publish.yml declares pub_credentials as a required secret:

secrets:
  pub_credentials:
    required: true

However, the workflow is documented as using automated publishing via OIDC, the inline comment marks the secret as deprecated, and the step that consumes it is already guarded:

- name: 🔐 Setup Pub Credentials
  if: ${{ secrets.pub_credentials != '' }}

Because the secret is required: true, callers following the documented example (no secrets: block) fail workflow validation, so OIDC-only publishing isn't possible without passing a dummy secret.

Affects v2 (v2.0.3) and main.

Steps to reproduce

jobs:
  publish:
    permissions:
      id-token: write
    uses: VeryGoodOpenSource/very_good_workflows/.github/workflows/dart_pub_publish.yml@v2
    with:
      working_directory: packages/my_package

Expected behavior

The workflow runs and publishes using the OIDC token provisioned by dart-lang/setup-dart.

Proposed fix

     secrets:
       pub_credentials:
-        required: true
+        required: false

Optionally, also expose an environment input (forwarded to the job's environment: so callers can use GitHub deployment environments as recommended by dart.dev.