Gate Docker login and snstage image usage on streamnativebot actor

[Copilot]

The Docker login and private snstage/pulsar-all image were unconditionally required for all CI runs. This gates both on github.actor == 'streamnativebot', allowing external contributors to run CI against the public apachepulsar/pulsar-all image without credentials.

• Dockerfile: Accept configurable base image via PULSAR_IMAGE build arg, defaulting to apachepulsar/pulsar-all
• run-integration-tests.sh: Forward PULSAR_IMAGE env var as --build-arg to docker build
• CI workflows: Conditionally set PULSAR_IMAGE per step and gate Docker login on actor

- name: Login SN docker hub
  if: github.actor == 'streamnativebot'
  run: docker login -u="${{ secrets.DOCKER_USER }}" -p="${{ secrets.DOCKER_PASSWORD}}"
- name: Run tests
  run: scripts/run-integration-tests.sh
  env:
    PULSAR_IMAGE: ${{ github.actor == 'streamnativebot' && 'snstage/pulsar-all' || 'apachepulsar/pulsar-all' }}

• Pulsar version: Updated default from 4.1.0.10 to 4.1.3

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[github-actions]

@Copilot:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)

[github-actions]

@Copilot:Thanks for your contribution. For this PR, do we need to update docs?
(The PR template contains info about doc, which helps others know more about the changes. Can you provide doc-related info in this and future PR descriptions? Thanks)