feat(core): capture Android locate UI trees

[quanruzhuoxiu]

Summary

• capture Android UI trees only for Locate UI contexts and preserve capture failures as non-fatal uiTreeError
• retain only the located target and its direct ancestor chain up to the nearest resource-id ancestor
• persist the retained tree directly on ExecutionTask.uiContext and in report dumps
• keep cache generation and replay behavior unchanged

Scope

This is PR 2 of 3 and depends on #2873. It contains no report UI and no XPath generation API.

The removed Agent.getXpathsByPoint(), XPath candidate generation, policy metadata, and uniqueness evidence are preserved on archive/agent-get-xpaths-by-point.

Stack

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

Validation

• pnpm run lint
• pnpm run type-check:tests
• pnpm exec nx build @midscene/core
• focused Core UI tree tests (12 tests passed)
• pnpm exec nx test @midscene/android (350 tests passed)

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