[APM] Add kibana.alert.grouping to error count threshold alerts

[fkanout]

Summary

Fixes #224901

Implements kibana.alert.grouping for the APM Error count threshold rule (apm.error_rate) as part of the Observability grouping initiative.

What changed

• Added kibana.alert.grouping to active alert payloads in the error count executor.
• Updated recovered alert context to prefer kibana.alert.grouping from the recovered alert document, with a backward-compatible fallback to reconstructed grouping for older alerts.
• Added dynamic template mapping for kibana.alert.grouping.* (string fields as keyword) in APM rule alert mappings.
• Updated unit and deployment-agnostic API integration tests to validate grouping behavior.

Why

This makes grouping first-class in alert documents for filtering/search/autocomplete and aligns recovered context.grouping behavior with the expected source of truth (kibana.alert.grouping).

Test plan

• yarn test:jest x-pack/solutions/observability/plugins/apm/server/routes/alerts/rule_types/error_count/register_error_count_rule_type.test.ts
• yarn test:ftr --config x-pack/solutions/observability/test/api_integration_deployment_agnostic/configs/stateful/oblt.apm.stateful.config.ts --grep "error count threshold alert"

Validation results

• Unit test suite: PASS (9/9)
• Deployment-agnostic APM API integration scenario: PASS (11 passing)

Notes

• No saved object migration required.
• Mapping change is additive and scoped to kibana.alert.grouping.*.

[elasticmachine]

Pinging @elastic/actionable-obs-team (Team:actionable-obs)

[elasticmachine]

Pinging @elastic/obs-presentation-team (Team:obs-presentation)

[elasticmachine]

💔 Build Failed

• Buildkite Build
• Commit: 2ad7e57

Failed CI Steps

• Jest Tests #1
• Jest Tests #1
• Post-Build

Test Failures

• [job] [logs] Jest Tests #1 / Condition entry input selecting Hash md5, sha1, or sha256 should call onChange with process.hash.*
• [job] [logs] Jest Tests #1 / TextareaInputArgument component should only send user input for display (valueText) to console when popup is closed

Metrics [docs]

‼️ ERROR: no builds found for mergeBase sha [48ac43a]

History

• 💔 Build #400609 failed ff7ecf4

cc @fkanout