[refurb] Correctly handle lengths of literal strings in slice-to-remove-prefix-or-suffix (FURB188) (#16237)

Fixes false negative when slice bound uses length of string literal.

We were meant to check the following, for example. Given:

```python
  text[:bound] if text.endswith(suffix) else text
```
We want to know whether:
   - `suffix` is a string literal and `bound` is a number literal
   - `suffix` is an expression and `bound` is
       exactly `-len(suffix)` (as AST nodes, prior to evaluation.)
       
The issue is that negative number literals like `-10` are stored as
unary operators applied to a number literal in the AST. So when `suffix`
was a string literal but `bound` was `-len(suffix)` we were getting
caught in the match arm where `bound` needed to be a number. This is now
fixed with a guard.


Closes #16231

crates/ruff_linter/resources/test/fixtures/refurb/FURB188.py

@@ -197,4 +197,11 @@ def handle_surrogates():
     text = "\ud800\udc00heythere"
     if text.startswith("\ud800\udc00"):
         text = text[1:]
-    
+
+# Regression test for
+# https://github.com/astral-sh/ruff/issues/16231
+def func():
+    a = "sjdfaskldjfakljklfoo"
+    if a.endswith("foo"):
+        a = a[: -len("foo")]
+    print(a)

crates/ruff_linter/src/rules/refurb/rules/slice_to_remove_prefix_or_suffix.rs

@@ -365,7 +365,7 @@ fn affix_matches_slice_bound(data: &RemoveAffixData, semantic: &SemanticModel) -
                 range: _,
                 value: string_val,
             }),
-        ) => operand.as_number_literal_expr().is_some_and(
+        ) if operand.is_number_literal_expr() => operand.as_number_literal_expr().is_some_and(
             |ast::ExprNumberLiteral { value, .. }| {
                 // Only support prefix removal for size at most `u32::MAX`
                 value

crates/ruff_linter/src/rules/refurb/snapshots/ruff_linter__rules__refurb__tests__FURB188_FURB188.py.snap

@@ -313,3 +313,23 @@ FURB188.py:193:5: FURB188 [*] Prefer `str.removeprefix()` over conditionally rep
 195 194 |     
 196 195 |     # should not be linted
 197 196 |     text = "\ud800\udc00heythere"
+
+FURB188.py:205:5: FURB188 [*] Prefer `str.removesuffix()` over conditionally replacing with slice.
+    |
+203 |   def func():
+204 |       a = "sjdfaskldjfakljklfoo"
+205 | /     if a.endswith("foo"):
+206 | |         a = a[: -len("foo")]
+    | |____________________________^ FURB188
+207 |       print(a)
+    |
+    = help: Use removesuffix instead of assignment conditional upon endswith.
+
+ℹ Safe fix
+202 202 | # https://github.com/astral-sh/ruff/issues/16231
+203 203 | def func():
+204 204 |     a = "sjdfaskldjfakljklfoo"
+205     |-    if a.endswith("foo"):
+206     |-        a = a[: -len("foo")]
+    205 |+    a = a.removesuffix("foo")
+207 206 |     print(a)