Generate custom JSON schema for dynamic setting (#9632)

## Summary

If you paste in the TOML for our default configuration (from the docs),
it's rejected by our JSON Schema:

![Screenshot 2024-01-23 at 10 08
09 PM](https://github.com/astral-sh/ruff/assets/1309177/7b4ea6e8-07db-4590-bd1e-73a01a35d747)

It seems like the issue is with:

```toml
# Set the line length limit used when formatting code snippets in
# docstrings.
#
# This only has an effect when the `docstring-code-format` setting is
# enabled.
docstring-code-line-length = "dynamic"
```

Specifically, since that value uses a custom Serde implementation, I
guess Schemars bails out? This PR adds a custom representation to allow
`"dynamic"` (but no other strings):

![Screenshot 2024-01-23 at 10 27
21 PM](https://github.com/astral-sh/ruff/assets/1309177/ab7809d4-b077-44e9-8f98-ed893aaefe5d)

This seems like it should work but I don't have a great way to test it.

Closes #9630.

crates/ruff_python_formatter/src/options.rs

@@ -1,10 +1,11 @@
+use std::fmt;
+use std::path::Path;
+use std::str::FromStr;
+
 use ruff_formatter::printer::{LineEnding, PrinterOptions, SourceMapGeneration};
 use ruff_formatter::{FormatOptions, IndentStyle, IndentWidth, LineWidth};
 use ruff_macros::CacheKey;
 use ruff_python_ast::PySourceType;
-use std::fmt;
-use std::path::Path;
-use std::str::FromStr;
 
 /// Resolved options for formatting one individual file. The difference to `FormatterSettings`
 /// is that `FormatterSettings` stores the settings for multiple files (the entire project, a subdirectory, ..)
@@ -367,15 +368,39 @@ impl fmt::Display for DocstringCode {
 #[cfg_attr(feature = "serde", serde(untagged))]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 pub enum DocstringCodeLineWidth {
+    /// Wrap docstring code examples at a fixed line width.
     Fixed(LineWidth),
+
+    /// Respect the line length limit setting for the surrounding Python code.
     #[default]
     #[cfg_attr(
         feature = "serde",
         serde(deserialize_with = "deserialize_docstring_code_line_width_dynamic")
     )]
+    #[cfg_attr(feature = "schemars", schemars(with = "DynamicSchema"))]
     Dynamic,
 }
 
+/// A dummy type that is used to generate a schema for `DocstringCodeLineWidth::Dynamic`.
+#[cfg(feature = "schemars")]
+struct DynamicSchema;
+
+#[cfg(feature = "schemars")]
+impl schemars::JsonSchema for DynamicSchema {
+    fn schema_name() -> String {
+        "Dynamic".to_string()
+    }
+
+    fn json_schema(_: &mut schemars::gen::SchemaGenerator) -> schemars::schema::Schema {
+        schemars::schema::SchemaObject {
+            instance_type: Some(schemars::schema::InstanceType::String.into()),
+            const_value: Some("dynamic".to_string().into()),
+            ..Default::default()
+        }
+        .into()
+    }
+}
+
 impl fmt::Debug for DocstringCodeLineWidth {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
         match *self {