RUM-18325: Restore accessibility-change dedup for full ViewEvent writes

[hamorillo]

What does this PR do?

Restores RumViewEventFilter's batch-level dedup for full ViewEvent writes, which was unintentionally disabled when RumDataWriter.hasAccessibility started always evaluating to true. Tracks the last-written accessibility snapshot per viewId in RumDataWriter, so hasAccessibility correctly reflects whether accessibility actually changed since the last full-view write for that specific view.

Motivation

Flagged by Codex's automated review on PR #3809: RumViewScope now always populates ViewEvent.view.accessibility (superseded AccessibilitySnapshotManager's null-unless-changed approach with diff-level dedup for ViewUpdateEvent, which works correctly). However, RumDataWriter still computed hasAccessibility = event.view.accessibility != null, which is now always true. Since RumViewEventFilter treats hasAccessibility == true as "always retain this event," dedup for redundant full ViewEvents never triggers anymore — every full view write is persisted/uploaded, even when a later document version fully supersedes it. Data volume only, not data loss (see RUM-18325 for full analysis).

Additional Notes

• We expect to remove this once partial view updates (and the periodic full-view checkpoint mechanism) become the only way full ViewEvents are written — at that point each view will only ever produce an initial and a closing full ViewEvent, and RumViewEventFilter's existing max-documentVersion dedup already covers that case without needing this exemption.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[datadog-datadog-prod-us1-2]

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• Patch Coverage: 100.00%
• Overall Coverage: 71.31% (-0.05%)

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-01T15:30:35.344174Z	1a3a46f	Draft marked ready

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