[go_router] Fix ShellRoute chrome dropped from semantics tree by ModalBarrier (#12353)

Shell chrome painted before a `ShellRoute` or `StatefulShellRoute` navigator (a side rail, or an app bar in a `Row`/`Column` based shell) disappears from the semantics tree. Screen readers cannot reach it at all: the nodes are not merely unnamed, they do not exist.

The mechanism: every `ModalRoute` builds a `ModalBarrier` wrapped in `BlockSemantics`, which drops the semantics of everything painted before it up to the nearest semantics boundary. The nested `Navigator` that go_router builds for shell routes does not establish such a boundary, so the block escapes the shell's navigator and prunes the shell's own chrome. Bottom-nav shells are unaffected only because `Scaffold` happens to paint its body before its bars.

This PR wraps the navigator built for `ShellRoute`/`StatefulShellRoute` branches in `Semantics(container: true)`, which contains the block. The root navigator is left unwrapped, since it has no earlier-painted siblings by construction. This is the workaround a framework team member confirmed on the linked issue; applying it inside go_router fixes it for every shell consumer without app-side patches.

Semantics tree of a minimal repro (a `Row` shell: 220px sidebar with three nav buttons, routed content on the right), before and after, captured with `debugDumpSemanticsTree`:

<details>
<summary>Before: 6 nodes, the entire sidebar subtree is missing</summary>

```
SemanticsNode#0
 │ Rect.fromLTRB(0.0, 0.0, 2400.0, 1800.0)
 │
 └─SemanticsNode#1
   │ Rect.fromLTRB(0.0, 0.0, 800.0, 600.0) scaled by 3.0x
   │ textDirection: ltr
   │ sortKey: OrdinalSortKey#39327(order: 0.0)
   │
   └─SemanticsNode#2
     │ Rect.fromLTRB(0.0, 0.0, 800.0, 600.0)
     │ flags: scopesRoute
     │
     └─SemanticsNode#3
       │ Rect.fromLTRB(221.0, 0.0, 800.0, 600.0)
       │ sortKey: OrdinalSortKey#39327(order: 0.0)
       │
       └─SemanticsNode#4
         │ Rect.fromLTRB(0.0, 0.0, 579.0, 600.0)
         │ flags: scopesRoute
         │
         └─SemanticsNode#5
             Rect.fromLTRB(85.5, 284.0, 493.5, 316.0)
             label: "Dashboard content"
             textDirection: ltr
```

Node `#3` starts at `x=221`, right of the 220px sidebar plus a 1px divider. There is no node anywhere for the sidebar: no title, no navigation container, no buttons. The sidebar is painted before the `/dashboard` route (`#4`, `scopesRoute`) inside the same enclosing semantics scope, which is exactly what that route's `BlockSemantics` drops.

</details>

<details>
<summary>After: sidebar fully present, routed content unchanged (two nodes for the repro's own toggle switch omitted for brevity)</summary>

```
SemanticsNode#0
 │ Rect.fromLTRB(0.0, 0.0, 2400.0, 1800.0)
 │
 └─SemanticsNode#1
   │ Rect.fromLTRB(0.0, 0.0, 800.0, 600.0) scaled by 3.0x
   │ textDirection: ltr
   │ sortKey: OrdinalSortKey#39327(order: 0.0)
   │
   └─SemanticsNode#2
     │ Rect.fromLTRB(0.0, 0.0, 800.0, 600.0)
     │ flags: scopesRoute
     │
     ├─SemanticsNode#3
     │   Rect.fromLTRB(16.0, 16.0, 204.0, 76.0)
     │   label: "Nested Navigator Semantics"
     │   textDirection: ltr
     │
     ├─SemanticsNode#4
     │ │ Rect.fromLTRB(0.0, 92.0, 220.0, 260.0)
     │ │ label: "Main navigation"
     │ │ textDirection: ltr
     │ │
     │ ├─SemanticsNode#5
     │ │   Rect.fromLTRB(0.0, 0.0, 220.0, 56.0)
     │ │   actions: focus, tap
     │ │   flags: isSelected, isButton, hasEnabledState, isEnabled,
     │ │     isFocusable, hasSelectedState
     │ │   label: "Dashboard"
     │ │   textDirection: ltr
     │ │
     │ ├─SemanticsNode#6
     │ │   Rect.fromLTRB(0.0, 56.0, 220.0, 112.0)
     │ │   actions: focus, tap
     │ │   flags: isButton, hasEnabledState, isEnabled, isFocusable,
     │ │     hasSelectedState
     │ │   label: "Settings"
     │ │   textDirection: ltr
     │ │
     │ └─SemanticsNode#7
     │     Rect.fromLTRB(0.0, 112.0, 220.0, 168.0)
     │     actions: focus, tap
     │     flags: isButton, hasEnabledState, isEnabled, isFocusable,
     │       hasSelectedState
     │     label: "Reports"
     │     textDirection: ltr
     │
     └─SemanticsNode#10
       │ Rect.fromLTRB(221.0, 0.0, 800.0, 600.0)
       │ sortKey: OrdinalSortKey#39327(order: 0.0)
       │
       └─SemanticsNode#11
         │ Rect.fromLTRB(0.0, 0.0, 579.0, 600.0)
         │ flags: scopesRoute
         │
         └─SemanticsNode#12
             Rect.fromLTRB(85.5, 284.0, 493.5, 316.0)
             label: "Dashboard content"
             textDirection: ltr
```

The routed content node ("Dashboard content") is byte-identical in both dumps. The fix does not change the routed content's semantics, only whether the chrome painted before the shell navigator survives alongside it.

</details>

Notes for review:

- Tests: the fix commit adds a `Shell navigator semantics boundary` group to `builder_test.dart` (chrome survives, structural wrap present, root navigator not wrapped), and a second commit adds a `StatefulShellRoute.indexedStack` regression test covering branch switching. Removing the wrap makes the chrome tests fail with `Found 0 widgets with a semantics label`.
- Version/CHANGELOG: go_router uses batch release, so this PR adds a file under `pending_changelogs/` (`version: patch`) instead of touching `pubspec.yaml` or `CHANGELOG.md`.
- Interaction with flutter/flutter#181519 (replacing `BlockSemantics` in modal routes with `AccessibilityFocusBlockType.blockSubtree`): the fix establishes a semantics container boundary at the shell navigator, which is where a nested navigator should scope its routes' blocking regardless of the blocking mechanism. If that migration later makes the containment unnecessary, the wrap stays harmless.

Fixes https://github.com/flutter/flutter/issues/135656
Related: https://github.com/flutter/flutter/issues/55758, https://github.com/flutter/flutter/issues/150978

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