feat(android): expose accessibility UI tree snapshots

[quanruzhuoxiu]

Summary

• parse Android accessibility hierarchy XML into typed UiNode snapshots
• expose AndroidDevice.getUITree() with a minimal snapshot containing platform, capturedAt, and root
• validate screenshot/package/bounds alignment and retry inconsistent captures within a bounded budget
• isolate collection, retry, and display-alignment helpers in ui-tree-capture.ts, leaving AndroidDevice.getUITree() as a thin entry point

Scope

This is PR 1 of 3. It contains Android snapshot collection only. Locate capture, task persistence, and the report UI are intentionally left to the dependent PRs. XPath generation is not part of the active stack.

The removed Agent.getXpathsByPoint() work is preserved on archive/agent-get-xpaths-by-point.

Stack

1. This PR: feat(android): expose accessibility UI tree snapshots #2873 — Android UI tree snapshot foundation
2. #2874 — Core Locate capture and target-lineage persistence
3. #2875 — Report UI tree viewer

Validation

• pnpm run lint
• pnpm exec nx build @midscene/core
• pnpm exec nx test @midscene/android (347 tests passed)
• pnpm exec nx build @midscene/android

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