fix(langfuse): correct nil check variable typo in extractModelInput

[SKDG042]

What type of PR is this?

fix

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• The description of this PR title is user-oriented and clear enough for others to understand.
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修正 extractModelInput 中 nil 检查的循环变量的使用错误，应当使用in而非ins

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:
In callbacks/langfuse/utils.go:38, the nil-check inside the for _, in := range ins loop was mistakenly comparing ins == nil (the slice itself) instead of in == nil (the current element). This makes the guard useless: when ins is nil, the loop body never executes; when ins is non-nil but contains nil elements, the guard always evaluates to false, and the subsequent in.Messages / in.Extra / in.Config accesses will panic on nil pointer dereference.

Before

for _, in := range ins {
    if ins == nil {   // bug: checks the slice, not the element
        continue
    }
    if len(in.Messages) > 0 { ... }  // panics if in is nil
}

After

  for _, in := range ins {
      if in == nil {    // correct: checks the current element
          continue
      }
      if len(in.Messages) > 0 { ... }
  }

zh(optional):
callbacks/langfuse/utils.go:38 中 for _, in := range ins 循环里的 nil 检查误将 in == nil（当前元素）写成了应该检查的 ins == nil（切片本身）。ins 为 nil 时循环根本不会进入；ins 非 nil 但含 nil 元素时，该判断恒为 false，后续 in.Messages 等访问会因空指针解引用 panic。

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

(optional) The PR that updates user documentation: