[unified_analytics] Tests for rootTool resolution, consent, and event fields #2703

Depends on #2699, #2700, #2701, #2702.

Cover:

• rootTool resolution: env var set, env var unset, invalid value, a shared-component label in DASH__TOOL (rejected), fallbackRootTool, and the test override.
• rootTool falling back to tool for top-level tools (dart-tool, flutter-tool, devtools).
• Shared components with no rootTool: analytics disabled, no consent message, no config write.
• Opt-in, consent message, and config writes keyed only on rootTool.
• root_tool and leaf_tool present on every event, and the deprecated tool property keeps today's value (the root tool for shared components, otherwise the tool itself).

Then publish a new unified_analytics version.