[web] Stop percent-decoding URLs written to browser history

[apinilabs-pascal]

When the framework reports new route information via the routeInformationUpdated
navigation message, the web engine rebuilds the URL and runs Uri.decodeComponent
over the whole string before handing it to the browser history (introduced in
flutter-team-archive/engine#40250). This strips one level of percent-encoding from the address
bar on every report:

Framework reports	Address bar before	Address bar after this PR
/item/foo%2Fbar	/item/foo/bar — refresh/share resolves to a different route	/item/foo%2Fbar
?q=x%26y	?q=x&y — value splits into two query parameters	?q=x%26y
?color=%23ffffff	?color=#ffffff — rest of the URL becomes a fragment	?color=%23ffffff

Because routers decode path parameters exactly once, this also made
GoRouterState.pathParameters differ between in-app navigation and a subsequent
browser refresh of the same URL: each round trip through the engine silently
dropped one encoding level.

Fix

handleNavigationMessage keeps the scheme/authority stripping, but now reassembles
the URL from the raw (still encoded) path, query, and fragment components
instead of decoding the result. The browser's address bar now reflects exactly what
the framework reported.

This supersedes #176029, which was closed for inactivity (missing tests). Unlike
that attempt, this PR also avoids reconstructing the query via
Uri(queryParameters:), which re-encoded values differently than reported
(e.g. %20 in a query value came back as +), and it adds a regression test.

Tests

Adds a regression test to lib/web_ui/test/engine/routing_test.dart asserting
that %20, %2F, and %26 in path, query, and fragment survive the round trip
into browser history unchanged. The existing routeInformationUpdated can handle uri test (scheme/authority stripping) is unaffected and still passes.

Fixes #180373
Fixes #171757
Fixes #147857
Fixes #155992

cc @mdebbar (you triaged #176029 and invited a re-open — this picks that up with tests)

Pre-launch Checklist

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• 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.
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[gemini-code-assist]

Code Review

This pull request modifies EngineFlutterWindow to preserve the percent-encoding of the path, query, and fragment when extracting the path from a URI string, and adds a corresponding regression test. Feedback on the changes points out that Dart's Uri properties (path, query, and fragment) return decoded strings, which would fail to preserve the percent-encoding and cause the new test to fail. It is suggested to extract these components directly from the original URI string instead.

[mdebbar]

Thanks for the contribution!

[yjbanov]

[auto-submit]

autosubmit label was removed for flutter/flutter/189835, because - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.

[mdebbar]

I had to fix something in internal Google Testing. It should be mergeable now. Thanks again for contributing this fix!