Include the opening brace's line in multiline f-string errors

An unclosed f-string replacement field can report an error on a later line, making the opening brace hard to find:

var = "abc"
a = f""" {var} extern "C" { """
b = """ string """

  File "script.py", line 3
    b = """ string """
                   ^
SyntaxError: f-string: expecting '}'

The quotes after the second { start a string inside the replacement field. That string ends on line 3, and the final quotes start another string that is never closed.

The error should include the line number of the opening brace:

SyntaxError: f-string: expecting '}' to close '{' on line 2

The same problem affects t-strings. Valid strings inside replacement fields should continue to work.

Linked PRs

• gh-157779

從提供的多行 f-string 重現程式碼開始，並將其診斷結果與預期訊息進行比較。檢查對應的 t-string 情況，並確認替換欄位中的有效字串仍然可以正常運作。完成的標準是：兩個受影響的錯誤訊息都指出左大括號所在的行，且不會導致有效字串回歸；一個已關聯的 pull request 已經列出。