Cache C-API prepared statements for repeated parameterized execute() calls

The pybind path was already patched with _get_or_prepare_pybind_statement
to reuse prepared statements for repeated parameterized execute()
calls, preventing unbounded memory growth (RSS raising).

The C-API execute() path had the same bug: every parameterized
execute() with a string query created a new PreparedStatement via
self._prepare(), which called self._connection.prepare() on the
C-API module — never reusing previously prepared statements.

Add _get_or_prepare_capi_statement() that caches PreparedStatement
objects keyed by the normalized query string in the same
_pybind_implicit_prepared_cache dict (already cleared on close).
The else branch now delegates to this cache instead of calling
self._prepare() directly.

Author: adsharma

src_py/connection.py

@@ -479,6 +479,18 @@ def _get_or_prepare_pybind_statement(
             self._pybind_implicit_prepared_cache[query] = prepared
         return prepared
 
+    def _get_or_prepare_capi_statement(
+        self,
+        query: str,
+        parameters: dict[str, Any],
+    ) -> PreparedStatement:
+        cached = self._pybind_implicit_prepared_cache.get(query)
+        if cached is not None:
+            return cached
+        prepared = self._prepare(query, parameters)
+        self._pybind_implicit_prepared_cache[query] = prepared
+        return prepared
+
     def _maybe_raise_scan_unsupported_object(self, query: str) -> None:
         match = re.search(
             r"\bLOAD\s+FROM\s+([A-Za-z_][A-Za-z0-9_]*)\b", query, re.IGNORECASE
@@ -582,7 +594,9 @@ def execute(
                     query, parameters
                 )
             prepared_statement = (
-                self._prepare(query, parameters) if isinstance(query, str) else query
+                self._get_or_prepare_capi_statement(query, parameters)
+                if isinstance(query, str)
+                else query
             )
             query_result_internal = self._connection.execute(
                 prepared_statement._prepared_statement, parameters