fix: allow page routes to opt out of route semantics - #182556

[haashem]

Nested navigators may use routes to update only part of the visible screen. However, Material and Cupertino page routes currently introduce a semantics route scope, causing screen readers to treat these updates as navigation to a new screen.

This adds includeRouteSemantics to:

• MaterialPageRoute
• MaterialPage
• CupertinoPageRoute
• CupertinoPage

The option defaults to true, preserving existing behavior. When set to false, the route content is not wrapped in a semantics node with scopesRoute: true.

Tests cover both imperative routes and declarative pages.

Fixes flutter/flutter#168915.

Pre-Review Checklist

• [✅] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [✅] 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.

[gemini-code-assist]

Code Review

This pull request adds an includeRouteSemantics option (defaulting to true) to CupertinoPageRoute, CupertinoPage, MaterialPageRoute, and MaterialPage, allowing them to opt out of introducing a semantics route scope. The review feedback suggests making the underlying _includeRouteSemantics getter public on both CupertinoRouteTransitionMixin and MaterialRouteTransitionMixin. This change would allow custom routes to override the property, simplify the overrides in the route and page classes, and enable reusing the documentation template via macros.