Add PKCE support for Tetrate Agent Router Service (#4165)

Signed-off-by: John Landa <jonathanlanda@gmail.com>
Co-authored-by: Michael Neale <michael.neale@gmail.com>

crates/goose/examples/tetrate_auth.rs

@@ -0,0 +1,39 @@
+// Example of Tetrate Agent Router Service PKCE authentication
+// Run with: cargo run --example tetrate_auth
+
+use goose::config::signup_tetrate::TetrateAuth;
+
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error>> {
+    println!("Testing Tetrate Agent Router Service PKCE flow...\n");
+
+    // Create new PKCE auth flow
+    let mut auth_flow = TetrateAuth::new()?;
+
+    // Get the auth URL that would be opened
+    let auth_url = auth_flow.get_auth_url();
+    println!("Auth URL: {}", auth_url);
+    println!("\nStarting authentication flow...");
+    println!("This will:");
+    println!("1. Open your browser to the auth page");
+    println!("2. Start a local server on port 3000");
+    println!("3. Wait for the callback\n");
+
+    // Complete the full flow
+    match auth_flow.complete_flow().await {
+        Ok(api_key) => {
+            println!("\n✅ Authentication successful!");
+            println!(
+                "API Key received: {}...",
+                &api_key.chars().take(10).collect::<String>()
+            );
+            println!("\nYou can now use this API key with the Tetrate provider.");
+        }
+        Err(e) => {
+            eprintln!("\n❌ Authentication failed: {}", e);
+            eprintln!("Error details: {:?}", e);
+        }
+    }
+
+    Ok(())
+}