Improve API exposed on ExprStringLiteral nodes (#16192)

## Summary

This PR makes the following changes:
- It adjusts various callsites to use the new
`ast::StringLiteral::contents_range()` method that was introduced in
https://github.com/astral-sh/ruff/pull/16183. This is less verbose and
more type-safe than using the `ast::str::raw_contents()` helper
function.
- It adds a new `ast::ExprStringLiteral::as_unconcatenated_literal()`
helper method, and adjusts various callsites to use it. This addresses
@MichaReiser's review comment at
https://github.com/astral-sh/ruff/pull/16183#discussion_r1957334365.
There is no functional change here, but it helps readability to make it
clearer that we're differentiating between implicitly concatenated
strings and unconcatenated strings at various points.
- It renames the `StringLiteralValue::flags()` method to
`StringLiteralFlags::first_literal_flags()`. If you're dealing with an
implicitly concatenated string `string_node`,
`string_node.value.flags().closer_len()` could give an incorrect result;
this renaming makes it clearer that the `StringLiteralFlags` instance
returned by the method is only guaranteed to give accurate information
for the first `StringLiteral` contained in the `ExprStringLiteral` node.
- It deletes the unused `BytesLiteralValue::flags()` method. This seems
prone to misuse in the same way as `StringLiteralValue::flags()`: if
it's an implicitly concatenated bytestring, the `BytesLiteralFlags`
instance returned by the method would only give accurate information for
the first `BytesLiteral` in the bytestring.

## Test Plan

`cargo test`

crates/ruff_python_formatter/src/expression/expr_string_literal.rs

@@ -28,9 +28,7 @@ impl FormatRuleWithOptions<ExprStringLiteral, PyFormatContext<'_>> for FormatExp
 
 impl FormatNodeRule<ExprStringLiteral> for FormatExprStringLiteral {
     fn fmt_fields(&self, item: &ExprStringLiteral, f: &mut PyFormatter) -> FormatResult<()> {
-        let ExprStringLiteral { value, .. } = item;
-
-        if let [string_literal] = value.as_slice() {
+        if let Some(string_literal) = item.as_unconcatenated_literal() {
             string_literal.format().with_options(self.kind).fmt(f)
         } else {
             // Always join strings that aren't parenthesized and thus, always on a single line.