[go_router] Fix chained top-level redirects not being fully resolved (#11108)

Fixes https://github.com/flutter/flutter/issues/178984

The `onEnter` refactor (commit `9ec29b6d23`, PR #8339) split the unified `redirect()` method into `applyTopLegacyRedirect()` (top-level, runs once) and `redirect()` (route-level only). This introduced two regressions:

1. **Top-level redirect chains broken**: `applyTopLegacyRedirect()` returned after one hop — a chain like `/ → /a → /b` stopped at `/a` instead of resolving to `/b`.
2. **Route-level → top-level chains broken**: `processRouteLevelRedirect()` recursed into `redirect()` without re-evaluating the top-level redirect on the new location.

### Fix

Unify top-level and route-level redirects back into `redirect()` as the single entry point for all redirect processing, while keeping the separation of concerns via `applyTopLegacyRedirect()` as an internal helper.

> **Note:** An earlier revision of this PR (v1) kept the split but added recursive calls in both methods. Based on [reviewer feedback](https://github.com/flutter/packages/pull/11108#discussion_r2943354003), the approach was refactored to move `applyTopLegacyRedirect()` inside `redirect()` so the caller (`parseRouteInformationWithDependencies`) doesn't need to know about the two-phase process.

#### Before (broken)

```mermaid
sequenceDiagram
    participant Parser as parseRouteInformationWithDependencies
    participant TopRedir as applyTopLegacyRedirect
    participant Redir as redirect (route-level only)

    Parser->>TopRedir: / (one hop only)
    TopRedir-->>Parser: /a (stops here ✗)
    Parser->>Redir: /a
    Redir->>Redir: route-level redirects
    Note right of Redir: No top-level re-evaluation
    Redir-->>Parser: result
```

#### After (fix)

```mermaid
sequenceDiagram
    participant Parser as parseRouteInformationWithDependencies
    participant Redir as redirect (unified)
    participant TopRedir as applyTopLegacyRedirect

    Parser->>Redir: / (initial matches)
    Redir->>TopRedir: / → top-level redirect chain
    TopRedir->>TopRedir: / → /a → /b (self-recursive)
    TopRedir-->>Redir: /b (fully resolved ✓)
    Redir->>Redir: route-level redirects on /b
    Note right of Redir: If route-level changes location,<br/>recurse → top-level re-evaluated
    Redir-->>Parser: final result
```

#### Key changes

- **`configuration.dart` — `redirect()`**: Now calls `applyTopLegacyRedirect()` first at every cycle, then processes route-level redirects on the post-top-level result. Route-level `_processRouteLevelRedirects` extracted as a helper.
- **`configuration.dart` — `applyTopLegacyRedirect()`**: Self-recursive to fully resolve top-level chains. No functional change from v1.
- **`parser.dart` — `parseRouteInformationWithDependencies()`**: Simplified — no longer calls `applyTopLegacyRedirect` separately. Just passes initial matches to `_navigate()`.
- **`parser.dart` — `_navigate()`**: Removed `preSharedHistory` parameter. Added `context.mounted` guard in the result `.then()` to protect the relocated async boundary.

Both fixes share the existing `redirectHistory` for loop detection and respect `redirectLimit`. The `onEnter` system is completely unaffected — it runs before redirects in the pipeline.

### Tests

- **19 redirect chain tests** (`redirect_chain_test.dart`): top-level chains, async chains, loop detection (including loop-to-initial), route→top cross-type chains, **async cross-type chains** (async top→route, async route→sync top, sync route→async top), **context disposal** during async top-level and route-level redirects, redirect limit boundary (exact limit succeeds, limit+1 fails), shared limit across redirect types.
- **3 onEnter interaction tests** (`on_enter_test.dart`): onEnter called once when chains resolve, onEnter block prevents redirect evaluation.
- **Full suite**: 418 tests pass, 0 regressions.

## Pre-Review Checklist

[^1]: This PR uses `pending_changelogs/` for versioning and changelog, following the go_router batch release process.