fix: reject trailing newline in tool-name validation

With re.match, a $-anchored pattern also matches just before a single
trailing newline, so tool-name validation accepted "name\n" without a
SEP-986 warning. Switch to re.fullmatch. Backport of #3076.

Co-authored-by: otiscuilei <otiscui@icloud.com>

Author: maxisbey

src/mcp/shared/tool_name_validation.py

@@ -77,7 +77,7 @@ def validate_tool_name(name: str) -> ToolNameValidationResult:
         warnings.append("Tool name starts or ends with a dot, which may cause parsing issues in some contexts")
 
     # Check for invalid characters
-    if not TOOL_NAME_REGEX.match(name):
+    if not TOOL_NAME_REGEX.fullmatch(name):
         # Find all invalid characters (unique, preserving order)
         invalid_chars: list[str] = []
         seen: set[str] = set()

tests/shared/test_tool_name_validation.py

@@ -66,12 +66,17 @@ def test_rejects_name_exceeding_max_length(self) -> None:
                 ("get,user,profile", "','"),
                 ("user/profile/update", "'/'"),
                 ("user@domain.com", "'@'"),
+                # a single trailing newline slipped past `$` with re.match
+                ("valid_name\n", "'\\n'"),
+                ("a" * 127 + "\n", "'\\n'"),
             ],
             ids=[
                 "with_spaces",
                 "with_commas",
                 "with_slashes",
                 "with_at_symbol",
+                "with_trailing_newline",
+                "max_length_with_trailing_newline",
             ],
         )
         def test_rejects_invalid_characters(self, tool_name: str, expected_char: str) -> None: