Disallow implicit concatenation of t-strings and other string types (#19485)

As of [this cpython PR](https://github.com/python/cpython/pull/135996),
it is not allowed to concatenate t-strings with non-t-strings,
implicitly or explicitly. Expressions such as `"foo" t"{bar}"` are now
syntax errors.

This PR updates some AST nodes and parsing to reflect this change.

The structural change is that `TStringPart` is no longer needed, since,
as in the case of `BytesStringLiteral`, the only possibilities are that
we have a single `TString` or a vector of such (representing an implicit
concatenation of t-strings). This removes a level of nesting from many
AST expressions (which is what all the snapshot changes reflect), and
simplifies some logic in the implementation of visitors, for example.

The other change of note is in the parser. When we meet an implicit
concatenation of string-like literals, we now count the number of
t-string literals. If these do not exhaust the total number of
implicitly concatenated pieces, then we emit a syntax error. To recover
from this syntax error, we encode any t-string pieces as _invalid_
string literals (which means we flag them as invalid, record their
range, and record the value as `""`). Note that if at least one of the
pieces is an f-string we prefer to parse the entire string as an
f-string; otherwise we parse it as a string.

This logic is exactly the same as how we currently treat
`BytesStringLiteral` parsing and error recovery - and carries with it
the same pros and cons.

Finally, note that I have not implemented any changes in the
implementation of the formatter. As far as I can tell, none are needed.
I did change a few of the fixtures so that we are always concatenating
t-strings with t-strings.

crates/ruff_python_codegen/src/generator.rs

@@ -1538,19 +1538,9 @@ impl<'a> Generator<'a> {
 
     fn unparse_t_string_value(&mut self, value: &ast::TStringValue) {
         let mut first = true;
-        for t_string_part in value {
+        for t_string in value {
             self.p_delim(&mut first, " ");
-            match t_string_part {
-                ast::TStringPart::Literal(string_literal) => {
-                    self.unparse_string_literal(string_literal);
-                }
-                ast::TStringPart::FString(f_string) => {
-                    self.unparse_interpolated_string(&f_string.elements, f_string.flags.into());
-                }
-                ast::TStringPart::TString(t_string) => {
-                    self.unparse_interpolated_string(&t_string.elements, t_string.flags.into());
-                }
-            }
+            self.unparse_interpolated_string(&t_string.elements, t_string.flags.into());
         }
     }