Remove ForestSummaryTrackingState

[CraigMacomber]

Description

Remove ForestSummaryTrackingState and derive it from trackedSummaryProperties===undefined instead.

This remove mutable state, and remove possible invaid states, removing the need for mutations and validation to maintain and ensure it is kept in sync.

Reviewer Guidance

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[Copilot]

Pull request overview

This PR removes the ForestSummaryTrackingState enum and derives the state from whether trackedSummaryProperties is undefined instead. This refactoring eliminates redundant mutable state and prevents potential invalid states where the two could become out of sync.

Changes:

• Removed the ForestSummaryTrackingState enum and associated type definition
• Replaced the forestSummaryState field with a derived isSummarizing getter
• Simplified validation functions by removing redundant state checks
• Updated all test assertions to use the new isSummarizing property

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
packages/dds/tree/src/feature-libraries/forest-summary/incrementalSummaryBuilder.ts	Removed ForestSummaryTrackingState enum, replaced forestSummaryState field with isSummarizing getter, simplified validation functions
packages/dds/tree/src/test/feature-libraries/forest-summary/incrementalSummaryBuilder.spec.ts	Updated all test assertions to use builder.isSummarizing instead of builder.forestSummaryState