Rollup merge of #150765 - ua/missing-colon, r=estebank

rustc_parse_format: improve error for missing `:` before `?` in format args

Detect the `{ident?}` pattern where `?` is immediately followed by `}` and emit a clearer diagnostic explaining that `:` is required for Debug formatting. This avoids falling back to a generic “invalid format string” error and adds a targeted UI test for the case.

Author: matthiaskrgr