feat: fix duplicate CI runs on pull requests

[Copilot]

The workflow triggered on both push and pull_request events, causing duplicate runs for PRs from same-repo branches.

Changes

• Restrict push trigger to master branch only
• Keep pull_request trigger for all PRs (same-repo and forks)

Before:

on: [push, pull_request]

After:

on:
  pull_request:
  push:
    branches:
      - master

PRs now show one set of checks instead of two (main / test (^16) (pull_request) + main / test (^16) (push)).

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[bug] needs to fix CI for strange duplicated checks in PR</issue_title>
<issue_description>The following duplicated CI checks ran in this open PR: #80

[main / test (^16) (pull_request)](https://github.com/node-casbin/mongoose-adapter/actions/runs/20065374084/job/57554620685?pr=80)
main / test (^16) (pull_request)Failing after 17s
[main / test (^16) (push)](https://github.com/node-casbin/mongoose-adapter/actions/runs/20065372994/job/57554620333?pr=80)
main / test (^16) (push)Failing after 17s
[main / test (^18) (pull_request)](https://github.com/node-casbin/mongoose-adapter/actions/runs/20065374084/job/57554620683?pr=80)
main / test (^18) (pull_request)Failing after 17s
[main / test (^18) (push)](https://github.com/node-casbin/mongoose-adapter/actions/runs/20065372994/job/57554620339?pr=80)
main / test (^18) (push)Failing after 20s

I think for an open state PR, it should not run "XXX (push)" checks, right? It should run for the git push after PR merge?
Need to fix the CI.</issue_description>

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

@hsluoyz @nodece @Shivansh-yadav13

• Fixes [bug] needs to fix CI for strange duplicated checks in PR #82

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[hsluoyz]

@nodece @Shivansh-yadav13 please review

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