[go_router] Add BlockedInitialNavigationException for blocked initial deep links

[davidmigloz]

When onEnter returns Block.stop() on the very first navigation attempt (e.g. a deep link on web, before any route has been committed), the parser's onCanNotEnter path has no prior configuration to restore and reports a generic GoException. Apps handling this in onException can't tell "initial deep link blocked" apart from other routing errors (unmatched routes, redirect loops) without string matching on the exception message.

This adds BlockedInitialNavigationException extends GoException, raised specifically in that no-prior-route path. It only refines the exception type — no change to when or whether an exception is thrown, and existing is GoException checks keep working. Apps can now write if (state.error is BlockedInitialNavigationException) in onException and, for example, redirect to a loading screen with the deep link preserved instead of treating it as a 404.

Test: widget test in on_enter_test.dart — initial navigation blocked by Block.stop() with an onException handler; asserts the captured state.error is a BlockedInitialNavigationException (and still a GoException).

Fixes flutter/flutter#189581

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[gemini-code-assist]

Code Review

This pull request introduces BlockedInitialNavigationException, a subtype of GoException, which is raised when initial navigation is blocked by onEnter without a prior route to restore. The parser is updated to throw this new exception, and a test case is added to verify this behavior. Feedback suggests removing the square brackets around Block in the documentation comments of BlockedInitialNavigationException to avoid unresolved doc reference warnings, since Block is not in scope in errors.dart.

[davidmigloz]

Heads-up: the onException recovery pattern this exception enables (redirecting away from a blocked initial deep link) only works end-to-end once #12216 lands, that one fixes an assertion crash when onException fires on the initial navigation (flutter/flutter#189582). The two are mergeable independently; landing #12216 first just means the typed exception ships with a working recovery path from day one.