Reject signed major in plugin manifest api version string (#1853)

Found while testing crates/plugin/src/manifest.rs:70 (api_from_number_or_string, the Api::Text arm)

Co-authored-by: Claude Haiku 5.5 <noreply@anthropic.com>

crates/plugin/src/manifest.rs

@@ -77,6 +77,7 @@ fn api_from_number_or_string<'de, D: serde::Deserializer<'de>>(d: D) -> Result<u
             .trim()
             .split('.')
             .next()
+            .filter(|major| !major.is_empty() && major.bytes().all(|b| b.is_ascii_digit()))
             .and_then(|major| major.parse().ok())
             .ok_or_else(|| serde::de::Error::custom(format!("invalid api version `{s}`"))),
     }

crates/plugin/src/tests.rs

@@ -472,3 +472,10 @@ fn api_may_be_a_version_string() {
     assert!(manifest(json!({"api": format!("{}.0.0", crate::API_VERSION + 1)})).is_err());
     assert!(manifest(json!({"api": "x"})).is_err());
 }
+
+#[test]
+fn api_version_string_rejects_signs() {
+    for bad in ["+2.0.0", "-1.0.0", "+2", ""] {
+        assert!(manifest(json!({"api": bad})).is_err(), "accepted {bad:?}");
+    }
+}