fix(core): handle parent/child mustache vars (#33345)

**Description:**

currently `mustache_schema("{{x.y}} {{x}}")` will error. pr fixes

**Issue:** na
**Dependencies:**na

---------

Co-authored-by: Bagatur <baskaryan@gmail.com>

libs/core/langchain_core/prompts/string.py

@@ -4,7 +4,7 @@ from __future__ import annotations
 
 import warnings
 from abc import ABC
-from collections.abc import Callable
+from collections.abc import Callable, Sequence
 from string import Formatter
 from typing import Any, Literal
 
@@ -149,9 +149,7 @@ def mustache_template_vars(
 Defs = dict[str, "Defs"]
 
 
-def mustache_schema(
+def mustache_schema(template: str) -> type[BaseModel]:
-    template: str,
-) -> type[BaseModel]:
     """Get the variables from a mustache template.
 
     Args:
@@ -175,6 +173,11 @@ def mustache_schema(
             fields[prefix] = False
         elif type_ in {"variable", "no escape"}:
             fields[prefix + tuple(key.split("."))] = True
+
+    for fkey, fval in fields.items():
+        fields[fkey] = fval and not any(
+            is_subsequence(fkey, k) for k in fields if k != fkey
+        )
     defs: Defs = {}  # None means leaf node
     while fields:
         field, is_leaf = fields.popitem()
@@ -327,3 +330,12 @@ class StringPromptTemplate(BasePromptTemplate, ABC):
     def pretty_print(self) -> None:
         """Print a pretty representation of the prompt."""
         print(self.pretty_repr(html=is_interactive_env()))  # noqa: T201
+
+
+def is_subsequence(child: Sequence, parent: Sequence) -> bool:
+    """Return True if child is subsequence of parent."""
+    if len(child) == 0 or len(parent) == 0:
+        return False
+    if len(parent) < len(child):
+        return False
+    return all(child[i] == parent[i] for i in range(len(child)))

libs/core/tests/unit_tests/prompts/test_string.py

@@ -0,0 +1,32 @@
+import pytest
+from packaging import version
+
+from langchain_core.prompts.string import mustache_schema
+from langchain_core.utils.pydantic import PYDANTIC_VERSION
+
+PYDANTIC_VERSION_AT_LEAST_29 = version.parse("2.9") <= PYDANTIC_VERSION
+
+
+@pytest.mark.skipif(
+    not PYDANTIC_VERSION_AT_LEAST_29,
+    reason=(
+        "Only test with most recent version of pydantic. "
+        "Pydantic introduced small fixes to generated JSONSchema on minor versions."
+    ),
+)
+def test_mustache_schema_parent_child() -> None:
+    template = "{{x.y}} {{x}}"
+    expected = {
+        "$defs": {
+            "x": {
+                "properties": {"y": {"default": None, "title": "Y", "type": "string"}},
+                "title": "x",
+                "type": "object",
+            }
+        },
+        "properties": {"x": {"$ref": "#/$defs/x", "default": None}},
+        "title": "PromptInput",
+        "type": "object",
+    }
+    actual = mustache_schema(template).model_json_schema()
+    assert expected == actual