refactor(route): remove unreachable backtracking branch for akind node

[Lcos-000]

What type of PR is this?

refactor

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• The description of this PR title is user-oriented and clear enough for others to understand.
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refactor(route): remove unreachable backtracking branch for akind node

(Optional) Translate the PR title into Chinese.

重构(route): 移除 any 节点回溯逻辑中的不可达分支

(Optional) More detailed description for this PR.

en:
While reading pkg/route/tree.go, the following logic was noticed in backtrackToNextNodeKind():

if previous.kind == akind {
nextNodeKind = skind
} else {
nextNodeKind = previous.kind + 1
}

The previous.kind == akind branch appears to be unreachable in the current implementation.

Evidence:

1. Control-flow: The only path entering an akind node exits the search loop immediately via break, so execution never reaches backtrackToNextNodeKind(akind).
2. Test verification: A panic guard was temporarily added in this branch and go test ./... -count=1 passed without triggering the panic.
3. History: This branch has existed since early versions, likely a defensive transition inherited from an earlier implementation.

Change: Removed the special case, simplifying to nextNodeKind = previous.kind + 1. No functional change. All existing tests pass.

zh(optional):
在阅读 pkg/route/tree.go 时，发现 backtrackToNextNodeKind() 中存在以下逻辑：

if previous.kind == akind {
nextNodeKind = skind
} else {
nextNodeKind = previous.kind + 1
}

其中 previous.kind == akind 分支在当前实现中似乎是不可达的。

分析：

1. 控制流：唯一进入 akind 节点的路径会立即通过 break 退出搜索循环，执行流程不会到达 backtrackToNextNodeKind(akind)。
2. 测试验证：在该分支中临时添加了 panic 守卫，运行 go test ./... -count=1 全部通过，panic 从未触发。
3. 历史追溯：该分支从早期版本就存在，可能是从更早实现中遗留的防御性过渡逻辑。

修改：移除特殊分支，简化为 nextNodeKind = previous.kind + 1。无功能变更，所有已有测试均通过。

(Optional) Which issue(s) this PR fixes:

Fixes #1504

(Optional) The PR that updates user documentation:

[Lcos-000]

After further consideration, I think a conservative approach might be better here — keep the original logic and just add a comment explaining why the akind branch is unreachable (see #1504 for analysis). This avoids any behavioral change on the hot path. Happy to update this PR or open a new one, whichever the maintainers prefer.