[performance]: Use hard coded langchain-core version to avoid importlib import (#30744)

This PR aims to reduce import time of `langchain-core` tools by removing
the `importlib.metadata` import previously used in `__init__.py`. This
is the first in a sequence of PRs to reduce import time delays for
`langchain-core` features and structures 🚀.

Because we're now hard coding the version, we need to make sure
`version.py` and `pyproject.toml` stay in sync, so I've added a new CI
job that runs whenever either of those files are modified. [This
run](https://github.com/langchain-ai/langchain/actions/runs/14358012706/job/40251952044?pr=30744)
demonstrates the failure that occurs whenever the version gets out of
sync (thus blocking a PR).

Before, note the ~15% of time spent on the `importlib.metadata` /related
imports

<img width="1081" alt="Screenshot 2025-04-09 at 9 06 15 AM"
src="https://github.com/user-attachments/assets/59f405ec-ee8d-4473-89ff-45dea5befa31"
/>

After (note, lack of `importlib.metadata` time sink):

<img width="1245" alt="Screenshot 2025-04-09 at 9 01 23 AM"
src="https://github.com/user-attachments/assets/9c32e77c-27ce-485e-9b88-e365193ed58d"
/>

libs/core/langchain_core/__init__.py

@@ -8,18 +8,13 @@ No third-party integrations are defined here. The dependencies are kept purposef
 very lightweight.
 """
 
-from importlib import metadata
-
 from langchain_core._api import (
     surface_langchain_beta_warnings,
     surface_langchain_deprecation_warnings,
 )
+from langchain_core.version import VERSION
 
-try:
-    __version__ = metadata.version(__package__)
-except metadata.PackageNotFoundError:
-    # Case where package metadata is not available.
-    __version__ = ""
+__version__ = VERSION
 
 surface_langchain_deprecation_warnings()
 surface_langchain_beta_warnings()

libs/core/langchain_core/version.py

@@ -0,0 +1,3 @@
+"""langchain-core version information and utilities."""
+
+VERSION = "0.3.51"