Add CloudHealth 2026-09-01-preview API version (#45711)

* Add CloudHealth 2026-09-01-preview API version

Introduce the 2026-09-01-preview version of Microsoft.CloudHealth,
sourced from the RPSaaSMaster branch of azure-rest-api-specs-pr.

Changes in this version:
- Add signal aggregation groups on entities (SignalAggregationGroup,
  signalAggregationGroups) with an optional displayName and optional
  aggregationType.
- Introduce the AggregationType union, adding the BestOf strategy.
- Rename DependenciesAggregationUnit to AggregationUnit and retype
  DependenciesSignalGroupV2.aggregationType/unit accordingly.
- Remove lookBackWindow from ThresholdRuleV2.
- Make DiscoveryRuleProperties.entityName optional.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* remove deleted status v09-01

* update dimensionFilter for 09

* remove the breaking change

* remove sentence

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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>