template: export GroupKey on template.Data

[locker95]

Pull Request Checklist

• Please list all open issue(s) discussed with maintainers related to this change
  • Fixes template.Data does not export GroupKey #2526
• Is this a new Receiver integration?
  • N/A
• Is this a bug fix?
  • I have added tests that can reproduce the bug which pass with this bug fix applied
• Is this a new feature?
  • N/A (small API completeness fix)
• Does this change affect performance?
  • N/A
• Is this a breaking change?
  • My changes do not break the existing cluster messages
  • My changes do not break the existing api
• I have added/updated the required documentation
• I have signed-off my commits
• I will follow best practices for contributing to this project

Summary

template.Data did not export GroupKey, even though the webhook JSON payload has always included groupKey. Webhook receivers that reuse template.Data (or custom templates that need the grouping key) had no way to access it without defining their own struct.

This change:

1. Adds GroupKey to template.Data with JSON tag groupKey (matching the documented webhook field).
2. Populates it from the notify context in GetTemplateData when a group key is present.
3. Adds unit tests for population and template access via {{ .GroupKey }}.

The webhook Message type still sets its own GroupKey field for the wire format; Go's encoding/json prefers the outer field, so the payload is unchanged.

Which user-facing changes does this PR introduce?

[ENHANCEMENT] template: Export GroupKey on template.Data for templates and webhook-style consumers. #2526

Notes from review discussion

• Custom webhook payload templates still will not see GroupKey until something like notify.DeepCopyWithTemplate edge cases #5012 (or renderPayload wiring) lands; this PR only exposes it on template.Data for the existing Go template path.
• groupKey is not globally unique (How to fix non unique GroupKeys? #3817); this change surfaces the dispatcher value as-is and does not claim uniqueness.

[coderabbitai]

📝 Walkthrough

Walkthrough

Adds GroupKey to template data, populates it from notification context, and tests propagation, template access, JSON naming, and empty behavior when absent.

Changes

GroupKey propagation

Layer / File(s)	Summary
Template data contract template/template.go	Adds the exported Data.GroupKey field with json:"groupKey" serialization.
Context propagation and validation notify/util.go, notify/util_test.go	Copies an available context group key into template data and tests propagation, template access, and missing-key behavior.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The change exports GroupKey and populates it in template.Data, matching issue #2526's request.
Out of Scope Changes check	✅ Passed	The diff stays focused on template.Data, notify propagation, and tests, with no unrelated changes evident.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Title check	✅ Passed	The title is concise, follows the required area: description format, and clearly reflects the exported GroupKey change.
Description check	✅ Passed	The description matches the template well, includes the checklist, a clear summary, and release notes for the user-facing change.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[TheMeier]

Just for the record:
one will not be able to use it in custom webhook payload templates see #5012. This could be mitigated by custom handling in renderPayload

Also the groupKey is not unique #3817.

[locker95]

@TheMeier Thanks for the notes — agreed on both points:

1. Custom webhook payload templates still won't see GroupKey until something like notify.DeepCopyWithTemplate edge cases #5012 / renderPayload wiring lands; this PR only exposes it on template.Data for the existing Go template path (email, etc.).
2. Non-uniqueness of groupKey (How to fix non unique GroupKeys? #3817) is pre-existing; this change just surfaces the value the dispatcher already uses, it doesn't claim global uniqueness.

I'll leave both caveats in mind if we extend this further (e.g. webhook payload). Happy to mention them in the PR description if useful.