language_model: Remove dependencies on individual model provider crates (#25503)

This PR removes the dependencies on the individual model provider crates
from the `language_model` crate.

The various conversion methods for converting a `LanguageModelRequest`
into its provider-specific request type have been inlined into the
various provider modules in the `language_models` crate.

The model providers we provide via Zed's cloud offering get to stay, for
now.

Release Notes:

- N/A

crates/language_models/src/provider/google.rs

@@ -272,7 +272,7 @@ impl LanguageModel for GoogleLanguageModel {
         request: LanguageModelRequest,
         cx: &App,
     ) -> BoxFuture<'static, Result<usize>> {
-        let request = request.into_google(self.model.id().to_string());
+        let request = into_google(request, self.model.id().to_string());
         let http_client = self.http_client.clone();
         let api_key = self.state.read(cx).api_key.clone();
 
@@ -303,7 +303,7 @@ impl LanguageModel for GoogleLanguageModel {
         'static,
         Result<futures::stream::BoxStream<'static, Result<LanguageModelCompletionEvent>>>,
     > {
-        let request = request.into_google(self.model.id().to_string());
+        let request = into_google(request, self.model.id().to_string());
 
         let http_client = self.http_client.clone();
         let Ok((api_key, api_url)) = cx.read_entity(&self.state, |state, cx| {
@@ -341,6 +341,38 @@ impl LanguageModel for GoogleLanguageModel {
     }
 }
 
+pub fn into_google(
+    request: LanguageModelRequest,
+    model: String,
+) -> google_ai::GenerateContentRequest {
+    google_ai::GenerateContentRequest {
+        model,
+        contents: request
+            .messages
+            .into_iter()
+            .map(|msg| google_ai::Content {
+                parts: vec![google_ai::Part::TextPart(google_ai::TextPart {
+                    text: msg.string_contents(),
+                })],
+                role: match msg.role {
+                    Role::User => google_ai::Role::User,
+                    Role::Assistant => google_ai::Role::Model,
+                    Role::System => google_ai::Role::User, // Google AI doesn't have a system role
+                },
+            })
+            .collect(),
+        generation_config: Some(google_ai::GenerationConfig {
+            candidate_count: Some(1),
+            stop_sequences: Some(request.stop),
+            max_output_tokens: None,
+            temperature: request.temperature.map(|t| t as f64).or(Some(1.0)),
+            top_p: None,
+            top_k: None,
+        }),
+        safety_settings: None,
+    }
+}
+
 pub fn count_google_tokens(
     request: LanguageModelRequest,
     cx: &App,