fix: reject trailing newline in tool-name and URI-template varname validation (#3076)

Python's $ with re.match also matches just before a single trailing newline, so tool-name validation accepted "name\n" and UriTemplate.parse accepted varnames like "foo\n". Switch both checks to re.fullmatch.

Closes #3084

tests/shared/test_uri_template.py

@@ -144,6 +144,8 @@ def test_parse_rejects_operator_without_variable():
         # RFC §2.3: dots only between varchars, not consecutive or trailing
         "foo..bar",
         "foo.",
+        # a single trailing newline slipped past `$` with re.match
+        "foo\n",
     ],
 )
 def test_parse_rejects_invalid_varname(name: str):