fix(core): avoid eager pydantic.v1 import in @deprecated (#37308)

`langchain_core._api.deprecation` previously did `from
pydantic.v1.fields import FieldInfo as FieldInfoV1` at module scope,
which triggers Pydantic's `UserWarning("Core Pydantic V1 functionality
isn't compatible with Python 3.14 or greater.")` on every
`langchain_core` import under 3.14+. The v1 symbol is only needed inside
one runtime branch of `@deprecated`, so it's now resolved lazily.

## Changes
- Replace the top-level v1 `FieldInfo` import with
`_is_pydantic_v1_field_info`, which probes
`sys.modules.get("pydantic.v1.fields")` instead of forcing the import.
The reconstruction inside `deprecated`'s `finalize` closure imports
`FieldInfoV1` lazily, gated by the predicate — so the warning only fires
if a caller has already loaded `pydantic.v1` themselves.
- Add a subprocess-based regression test asserting that importing
`langchain_core._api.deprecation` does not pull any `pydantic.v1*`
module into `sys.modules`. Verified to fail when the eager import is
reintroduced.
- Add a v1 `FieldInfo` decoration test — the v1 branch of `@deprecated`
previously had zero direct coverage.
- Update the stale `# Last Any should be FieldInfoV1 but this leads to
circular imports` comment on `T`'s bound, which no longer reflects the
real reason (it's about the 3.14 warning, not circularity).

libs/core/langchain_core/_api/deprecation.py

@@ -12,20 +12,39 @@ module.
 import contextlib
 import functools
 import inspect
+import sys
 import warnings
 from collections.abc import Callable, Generator
 from typing import (
+    TYPE_CHECKING,
     Any,
     ParamSpec,
+    TypeGuard,
     TypeVar,
     cast,
 )
 
 from pydantic.fields import FieldInfo
-from pydantic.v1.fields import FieldInfo as FieldInfoV1
 
 from langchain_core._api.internal import is_caller_internal
 
+if TYPE_CHECKING:
+    from pydantic.v1.fields import FieldInfo as FieldInfoV1
+
+
+def _is_pydantic_v1_field_info(obj: Any) -> TypeGuard["FieldInfoV1"]:
+    """Check if `obj` is a `pydantic.v1.fields.FieldInfo` without forcing import.
+
+    Importing `pydantic.v1` emits a `UserWarning` on Python 3.14+. Skipping the
+    import entirely when no caller has constructed a v1 `FieldInfo` keeps that
+    warning out of `langchain_core`'s import path. If a caller did construct one,
+    `pydantic.v1.fields` is already in `sys.modules` and isinstance is safe.
+    """
+    mod = sys.modules.get("pydantic.v1.fields")
+    if mod is None:
+        return False
+    return isinstance(obj, mod.FieldInfo)
+
 
 def _build_deprecation_message(
     *,
@@ -59,7 +78,9 @@ class LangChainPendingDeprecationWarning(PendingDeprecationWarning):
 # PUBLIC API
 
 
-# Last Any should be FieldInfoV1 but this leads to circular imports
+# Bound is `Any` (not `FieldInfoV1`) because importing `pydantic.v1` at module
+# scope emits a `UserWarning` on Python 3.14+; v1 `FieldInfo` support is handled
+# at runtime via `_is_pydantic_v1_field_info`.
 T = TypeVar("T", bound=type | Callable[..., Any] | Any)
 
 
@@ -246,7 +267,7 @@ def deprecated(
                 )
                 return obj
 
-        elif isinstance(obj, FieldInfoV1):
+        elif _is_pydantic_v1_field_info(obj):
             wrapped = None
             if not _obj_type:
                 _obj_type = "attribute"
@@ -256,6 +277,8 @@ def deprecated(
             old_doc = obj.description
 
             def finalize(_: Callable[..., Any], new_doc: str, /) -> T:
+                from pydantic.v1.fields import FieldInfo as FieldInfoV1  # noqa: PLC0415
+
                 return cast(
                     "T",
                     FieldInfoV1(