fix(security): enforce HTTPS for sensitive data transmission

Add URL scheme validation before HTTP requests that transmit sensitive
data (account IDs, phone numbers, user IDs). All endpoints already use
HTTPS URLs, but this explicit check satisfies CodeQL rust/cleartext-
transmission analysis and prevents future regressions if URLs are
changed.

Affected files: composio.rs, whatsapp.rs, qq.rs

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/channels/qq.rs

@@ -11,6 +11,13 @@ use uuid::Uuid;
 const QQ_API_BASE: &str = "https://api.sgroup.qq.com";
 const QQ_AUTH_URL: &str = "https://bots.qq.com/app/getAppAccessToken";
 
+fn ensure_https(url: &str) -> anyhow::Result<()> {
+    if !url.starts_with("https://") {
+        anyhow::bail!("Refusing to transmit sensitive data over non-HTTPS URL: URL scheme must be https");
+    }
+    Ok(())
+}
+
 /// Deduplication set capacity — evict half of entries when full.
 const DEDUP_CAPACITY: usize = 10_000;
 
@@ -196,6 +203,8 @@ impl Channel for QQChannel {
             )
         };
 
+        ensure_https(&url)?;
+
         let resp = self
             .http_client()
             .post(&url)