Fix selection registration for hidden overlay entries - #189713

OverlayEntrys that remain mounted behind an opaque entry still build their
subtrees to preserve state, but _RenderTheater skips their layout and paint.
Those hidden subtrees were still inheriting the ancestor selection registrar,
so their selectables could be registered and compared before they had been laid
out. That could trigger the RenderParagraph.debugNeedsLayout assertion while
selection ordering was flushed.

This wraps only hidden maintained entries in SelectionContainer.disabled when
an ancestor selection registrar is present. Onstage entries continue to inherit
the existing registrar, and the route subtree remains mounted throughout so
its state is preserved. The public selection registrar API is unchanged.

Fixes #119772.

Tests:

• ./bin/flutter test --no-pub packages/flutter/test/widgets/selectable_region_test.dart
• ../../bin/flutter test --no-pub --platform chrome test/widgets/selectable_region_test.dart --plain-name "ignores selection content in background Navigator pages" (from packages/flutter)
• ./bin/flutter test --no-pub packages/flutter/test/widgets/selection_container_test.dart packages/flutter/test/widgets/overlay_test.dart
• ./bin/flutter analyze --no-pub packages/flutter/lib/src/widgets/overlay.dart packages/flutter/lib/src/widgets/selection_container.dart packages/flutter/test/widgets/selectable_region_test.dart
• ./bin/dart format --output=none --set-exit-if-changed packages/flutter/lib/src/widgets/overlay.dart packages/flutter/lib/src/widgets/selection_container.dart packages/flutter/test/widgets/selectable_region_test.dart
• git diff --check

Risk:

• Two focused framework/test files are changed; no generated output is included.
• Existing SelectionContainer.disabled behavior is used to exclude hidden overlay entries from selection while preserving their mounted subtree. No public API is changed.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting a request.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

If this change needs to override an active code freeze, provide a comment explaining why. The Flutter team can override the code freeze workflow. See pinned issues for current guidance.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find comments useful, you may update the code accordingly, but if you are unsure or disagree, wait for a Flutter team member's review.