Respect word boundaries when detecting function signature in docs (#13388)

## Summary

Closes https://github.com/astral-sh/ruff/issues/13242.

crates/ruff_linter/src/rules/pydocstyle/mod.rs

@@ -49,6 +49,7 @@ mod tests {
     #[test_case(Rule::OverIndentation, Path::new("D.py"))]
     #[test_case(Rule::OverIndentation, Path::new("D208.py"))]
     #[test_case(Rule::NoSignature, Path::new("D.py"))]
+    #[test_case(Rule::NoSignature, Path::new("D402.py"))]
     #[test_case(Rule::SurroundingWhitespace, Path::new("D.py"))]
     #[test_case(Rule::DocstringStartsWithThis, Path::new("D.py"))]
     #[test_case(Rule::UnderIndentation, Path::new("D.py"))]

crates/ruff_linter/src/rules/pydocstyle/rules/no_signature.rs

@@ -66,7 +66,25 @@ pub(crate) fn no_signature(checker: &mut Checker, docstring: &Docstring) {
     // a function named `foo`).
     if first_line
         .match_indices(function.name.as_str())
-        .any(|(index, _)| first_line[index + function.name.len()..].starts_with('('))
+        .any(|(index, _)| {
+            // The function name must be preceded by a word boundary.
+            let preceded_by_word_boundary = first_line[..index]
+                .chars()
+                .next_back()
+                .map_or(true, |c| matches!(c, ' ' | '\t' | ';' | ','));
+            if !preceded_by_word_boundary {
+                return false;
+            }
+
+            // The function name must be followed by an open parenthesis.
+            let followed_by_open_parenthesis =
+                first_line[index + function.name.len()..].starts_with('(');
+            if !followed_by_open_parenthesis {
+                return false;
+            }
+
+            true
+        })
     {
         checker
             .diagnostics

crates/ruff_linter/src/rules/pydocstyle/snapshots/ruff_linter__rules__pydocstyle__tests__D402_D402.py.snap

@@ -0,0 +1,18 @@
+---
+source: crates/ruff_linter/src/rules/pydocstyle/mod.rs
+---
+D402.py:2:5: D402 First line should not be the function's signature
+  |
+1 | def foo():
+2 |     """Returns foo()."""
+  |     ^^^^^^^^^^^^^^^^^^^^ D402
+3 | 
+4 | def foo():
+  |
+
+D402.py:8:5: D402 First line should not be the function's signature
+  |
+7 | def foo():
+8 |     """"Use this function; foo()."""
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ D402
+  |