Fix Hero size changes during Navigator resize - #190025

Fixes #134647

Description

Hero flights were positioned by converting the evaluated flight Rect into a RelativeRect using the Navigator size captured when the flight started.

This caused the Hero's size or position to become distorted if the Navigator resized during the transition. It was especially visible with nested Navigators because they can be resized by their parent layout, for example when a Scaffold shrinks in response to the keyboard. The root Navigator's Overlay typically keeps the full viewport size, so the issue was less likely to occur there.

This change positions the Hero directly from the evaluated Rect, avoiding calculations based on stale Navigator dimensions.

A regression test was added that resizes a nested Navigator during a Hero flight and verifies that the shuttle preserves its size.

Demonstration

Pre-launch Checklist

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