[go_router] Document regex constraints for path parameters

[saurabh-mirajkar]

Description

Documents the regular expression constraint syntax supported by GoRoute.path.

GoRoute already supports path parameters in the form :paramName(regex), but this syntax is not currently documented. This change documents the syntax, provides a valid Dart example, and explains how constrained path parameters participate in route matching.

Fixes flutter/flutter#177766

Testing

• No new tests were added because this is a documentation-only change.
• Ran dart run script/tool/bin/flutter_plugin_tools.dart test --packages=go_router.

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• All existing and new tests are passing.

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Footnotes

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[flutter-dashboard]

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[gemini-code-assist]

Code Review

This pull request documents the support for regular expression constraints in GoRoute path parameters within route.dart and adds a corresponding pending changelog entry. There are no review comments, and no additional feedback is provided.