Trying out go_router support for OpenContainer

[Piinks]

Design Doc

This PR fixes flutter/flutter#121929 by making the OpenContainer widget compatible with declarative routers like go_router.

Problem

OpenContainer previously hardcoded an imperative Navigator.push call. When used with declarative routers on the web, this prevented the browser URL from updating when the container opened. Manually triggering a router navigation would lose the signature "morphing" animation because the destination page had no metadata about the source widget's position/size.

Solution

I have introduced a coordination mechanism that allows the source widget and the destination page to share animation metadata while letting the Router own the navigation state.

Key Changes

1. A new Page class that can be used in Router route definitions.
2. Added to OpenContainer to allow overriding the default imperative push with a declarative navigation call (e.g., context.go()).
3. A shared identifier (similar to Hero.tag) used to link the source OpenContainer with the destination OpenContainerPage.
4. An internal registry that coordinates the "morphing" measurements between the source and destination.
5. Updated OpenContainerRoute to handle cases where the source widget is not present (e.g., direct URL deep-linking) by falling back to a smooth fade transition.
6. Made OpenContainerRoute and Hideable public to support these new declarative workflows.

Usage Example

// List Page
OpenContainer(
  transitionTag: 'item-$id',
  onOpen: () => context.go('/details/$id'),
  closedBuilder: (context, action) => ListTile(onTap: action, ...),
  openBuilder: (context, action) => DetailsPage(id: id),
)

// Router Config
GoRoute(
  path: '/details/:id',
  pageBuilder: (context, state) => OpenContainerPage(
    transitionTag: 'item-${state.pathParameters['id']}',
    openBuilder: (context, action) => DetailsPage(id: id),
  ),
)

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter.
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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Footnotes

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