chore: Add AgentPlatform SDK langchain tests for python 3.12 and 3.13

PiperOrigin-RevId: 926671278

Author: sararob

.kokoro/presubmit/unit_langchain_py312.cfg

@@ -0,0 +1,13 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Run unit tests for LangChain on Python 3.12
+env_vars: {
+    key: "NOX_SESSION"
+    value: "unit_langchain-3.12"
+}
+
+# Run unit tests in parallel, splitting up by file
+env_vars: {
+    key: "PYTEST_ADDOPTS"
+    value: "-n=auto --dist=loadscope"
+}

.kokoro/presubmit/unit_langchain_py313.cfg

@@ -0,0 +1,13 @@
+# Format: //devtools/kokoro/config/proto/build.proto
+
+# Run unit tests for LangChain on Python 3.13
+env_vars: {
+    key: "NOX_SESSION"
+    value: "unit_langchain-3.13"
+}
+
+# Run unit tests in parallel, splitting up by file
+env_vars: {
+    key: "PYTEST_ADDOPTS"
+    value: "-n=auto --dist=loadscope"
+}