fixes #25821; unary minus off by one mistake [backport] (#25823)

fixes #25821

This pull request includes a minor bug fix in the lexer and adds new
test cases for string formatting with binary operators in interpolated
expressions.

Lexer bug fix:

* Fixed an off-by-one error in the unary minus detection logic in the
`rawGetTok` procedure in `lexer.nim`, ensuring that the start-of-buffer
condition is correctly checked.

Testing improvements:

* Added tests to `tstrformat.nim` to verify that binary operators (such
as subtraction) work correctly inside interpolated string expressions
using both `&` and `fmt`.

tests/stdlib/tstrformat.nim

@@ -544,6 +544,12 @@ proc main() =
     var x = 5
     doAssert fmt"{(x=7;123.456)=:13e}" == "(x=7;123.456)= 1.234560e+02"
     doAssert x==7
+
+  block: # binary operators in interpolated expressions
+    let n = 1
+    doAssert &"{n-1}" == "0"
+    doAssert fmt"{n-1}" == "0"
+
   block: #curly bracket expressions and tuples
     proc formatValue(result: var string; value:Table|bool|JsonNode; specifier:string) = result.add $value