[go_router] Fix relative navigation after imperative navigation

[Boulea7]

Relative navigation after an imperative push currently resolves against the underlying route information instead of the visible imperative route. This updates go, push, pushReplacement, and replace to resolve ./ locations against the imperative top match, while preserving the existing behavior for other locations. Regression coverage includes query parameters and fragments.

This revisits and completes the fix proposed in #11726. Thank you @NadeemIqbal for the earlier investigation and implementation.

Fixes flutter/flutter#182441.

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Code Review

This pull request updates GoRouter to resolve relative locations starting with ./ against the last imperatively-pushed route match for the go, push, pushReplacement, and replace methods, and adds corresponding regression tests. The review feedback suggests extending this resolution logic to also support relative paths starting with ../.