feat(pagerduty): add client and client_url support for Events API v2

[jyoti369]

What

Add optional client and client_url parameters to the PagerDuty Events API v2 integration, per the Events API v2 spec.

These fields allow a clickable "View in [client]" link to appear in PagerDuty event details.

Why

The current Events API implementation is missing client and client_url attributes. Without them, events created via Keep don't show the monitoring client link in PagerDuty.

Changes

• Added client and client_url as optional named parameters in _notify, _send_alert, and _build_alert
• Both fields are placed at the top level of the event payload (not inside payload.payload), matching the API spec
• Added unit tests covering:
  • client/client_url included when provided
  • client/client_url omitted when not provided
  • Full payload structure validation

Usage in workflows

actions:
  - name: pagerduty-alert
    provider:
      type: pagerduty
      config: "{{ providers.pagerduty }}"
    with:
      title: "CPU at 99%"
      severity: critical
      client: "Keep"
      client_url: "https://keep.example.com/alerts/123"

Closes #6233

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[jyoti369]

Unit test failure is unrelated to this PR - it's a flaky SQLite locking error during teardown in test_workflow_keep_notify_after_foreach:

sqlalchemy.exc.OperationalError: (sqlite3.OperationalError) database table is locked
[SQL: DROP TABLE workflowtoincidentexecution]

938 tests passed, only this one errored due to SQLite concurrency. Could you re-run the failing check?

[jyoti369]

@shahargl The images/links placement has been reverted back to the original nesting inside payload["payload"] as you requested. The PR now only adds the new client and client_url fields at top level (which is correct per the PagerDuty Events API v2 spec). All CI checks are green. Ready for re-review when you get a chance.

[bbcorp]

lgtm
Hey @shahargl can you have a look and confirm it looks good to you too please?