fix(oracle): preallocate price account serialization

Use borsh::object_length to reserve the exact serialized size for OraclePriceAccount data without relying on size_of_val.

Add coverage for string-backed source identifiers.

oracle/core/src/lib.rs

@@ -173,7 +173,9 @@ impl TryFrom<&Data> for OraclePriceAccount {
 
 impl From<&OraclePriceAccount> for Data {
     fn from(price_account: &OraclePriceAccount) -> Self {
-        let mut data = Vec::new();
+        let serialized_len =
+            borsh::object_length(price_account).expect("Oracle price account length must be known");
+        let mut data = Vec::with_capacity(serialized_len);
         BorshSerialize::serialize(price_account, &mut data)
             .expect("Serialization to Vec should not fail");
         Self::try_from(data).expect("Oracle price account encoded data should fit into Data")
@@ -292,6 +294,19 @@ mod tests {
         );
     }
 
+    #[test]
+    fn oracle_price_account_data_matches_borsh_object_length() {
+        let mut oracle_account = price_account(1_050_000, 1_000);
+        oracle_account.source_identifier = "mock-oracle-with-long-source-name".to_owned();
+
+        let data = Data::from(&oracle_account);
+
+        assert_eq!(
+            data.as_ref().len(),
+            borsh::object_length(&oracle_account).expect("object length should be known")
+        );
+    }
+
     #[test]
     fn account_price_feed_rejects_wrong_feed_account() {
         let oracle_account = price_account(1_050_000, 1_000);