feat: validate endpoint integration, Expr quantity sizing, apply_func input field fix

- Add /api/v1/strategies/validate client to SwymClient; wire into agent loop
  before submission so all DSL errors are surfaced in one round-trip
- Update dsl-schema.json to upstream: quantity is now oneOf[DecimalString, Expr],
  ExprApplyFunc uses "input" field (renamed from "expr")
- Update prompts: document expression-based quantity sizing (fixed-fraction and
  ATR-based examples), fix apply_func to use "input" not "expr" throughout
- Remove unused ValidationError import

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/swym.rs

@@ -4,6 +4,20 @@ use serde::{Deserialize, Serialize};
 use serde_json::Value;
 use uuid::Uuid;
 
+/// Response from `POST /api/v1/strategies/validate`.
+#[derive(Debug, Deserialize)]
+pub struct ValidationResponse {
+    pub valid: bool,
+    #[serde(default)]
+    pub errors: Vec<ValidationError>,
+}
+
+#[derive(Debug, Deserialize, Clone)]
+pub struct ValidationError {
+    pub path: String,
+    pub message: String,
+}
+
 /// Client for the swym backtesting API.
 pub struct SwymClient {
     client: Client,
@@ -254,6 +268,32 @@ impl SwymClient {
         resp.json().await.context("parse candle coverage")
     }
 
+    /// Validate a strategy against the swym DSL schema.
+    ///
+    /// Calls `POST /api/v1/strategies/validate` and returns a structured list
+    /// of all validation errors. Returns `Ok(vec![])` when the strategy is valid.
+    /// Returns `Err` only on network or parse failures, not on DSL errors.
+    pub async fn validate_strategy(&self, strategy: &Value) -> Result<Vec<ValidationError>> {
+        let url = format!("{}/strategies/validate", self.base_url);
+        let resp = self
+            .client
+            .post(&url)
+            .json(strategy)
+            .send()
+            .await
+            .context("validate strategy request")?;
+
+        if !resp.status().is_success() {
+            let status = resp.status();
+            let body = resp.text().await.unwrap_or_default();
+            anyhow::bail!("validate strategy {status}: {body}");
+        }
+
+        let parsed: ValidationResponse =
+            resp.json().await.context("parse validation response")?;
+        Ok(parsed.errors)
+    }
+
     /// Submit a backtest run.
     pub async fn submit_backtest(
         &self,