Make serialization backwards-compatible for collab server (#27626)

Sets up the collab server to accept the format of system message that
we'll introduce later for [prompt
caching](https://docs.anthropic.com/en/docs/build-with-claude/prompt-caching).

Release Notes:

- N/A

crates/anthropic/src/anthropic.rs

@@ -585,6 +585,13 @@ pub enum Thinking {
     Enabled { budget_tokens: Option<u32> },
 }
 
+#[derive(Debug, Serialize, Deserialize)]
+#[serde(untagged)]
+pub enum StringOrContents {
+    String(String),
+    Content(Vec<RequestContent>),
+}
+
 #[derive(Debug, Serialize, Deserialize)]
 pub struct Request {
     pub model: String,
@@ -597,7 +604,7 @@ pub struct Request {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub tool_choice: Option<ToolChoice>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
-    pub system: Option<String>,
+    pub system: Option<StringOrContents>,
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub metadata: Option<Metadata>,
     #[serde(default, skip_serializing_if = "Vec::is_empty")]