refactor: remove unused findChild method from node

[lxcxjxhx]

What

Remove the findChild method from node and replace its 3 call sites with findChildWithLabel.

Why

findChild is a strict subset of findChildWithLabel — it only searches children, while findChildWithLabel also checks paramChild and anyChild. All 3 call sites search for '/' or other non-param/any characters, so the behavior is identical. Keeping both methods adds confusion without benefit.

Changes

• tree.go: delete findChild method (8 lines)
• tree.go: replace 3 findChild calls with findChildWithLabel

Testing

go test ./pkg/route/...

All existing tests pass. This is a pure refactor with no behavior change.

Closes #1504

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