fix(langchain): fix patching langchain throws an ImportError [backport 3.15] (#14739)

Backport
967ff6d
from #14687 to 3.15.

## Description

Changes our patching logic to only patch `langchain_core`, as we have
not supported operations on `langchain` directly for some time, and is
being removed in all langchain 1.0.0 releases - patching this was
causing `ImporError`s due to circular imports from how langchain
switched some imports from inlined to top-level.

The reason we patched `langchain-community`, `langchain-openai`, and
`langchain-pinecone` in the first place was because we could not patch
the underlying `embed_query`, `embed_documents`, and `similarity_search`
for embeddings and vectorstores as the base classes are abstract.
However, by patching `__init_subclass__`, we can successfully patch them
on each instance, meaning we only need to patch `langchain_core`.

This is reflected in the change in `_monkey.py`, and the imported
`langchain_core` in the patching file.

## Testing

Updated all riot lockfiles and ensured existing tests continued to pass.
In order to make tests work (as `__init_submodules__` did not play
nicely with cached modules & patching/unpatching every test), the
`langchain_core` fixture is scoped to each module.

Additionally, running this script:

```python
from ddtrace.llmobs import LLMObs
LLMObs.enable(ml_app="sam-test")

from langchain_openai import ChatOpenAI

chat = ChatOpenAI(
    model = "gpt-3.5-turbo",
    temperature=0.1,
)
resp = chat.invoke("What is the capital of France?")
```

with

```bash
$ python -m venv .venv
$ source .venv/bin/activate
$ pip install langchain_openai
$ pip install ddtrace
```

Should not experience any spans being submitted, but with

```bash
$ pip install -e /path/to/dd-trace-py
```

checked out to this branch should see spans being submitted.


[Trace](https://dd.datad0g.com/llm/traces?query=%40ml_app%3Asam-test%20%40event_type%3Aspan%20%40parent_id%3Aundefined&agg_m=count&agg_m_source=base&agg_t=count&fromUser=false&sp=%5B%7B%22sp%22%3A%7B%22width%22%3A%22min%28100%25%2C%20max%28calc%28100%25%20-%20var%28--ui-page-left-offset%29%20-%2016px%29%2C%20900px%29%29%22%7D%2C%22p%22%3A%7B%22eventId%22%3A%22AwAAAZl4GYUiibFcWwAAABhBWmw0R1lVaUFBQVI1bGtTMlMtaUFBQUEAAAAkZDE5OTc4MTktYTZkYS00NzMxLTk1NTctZmZjNTk0MDc4NjM5AAAAGA%22%7D%2C%22i%22%3A%22llm-obs-panel%22%7D%5D&spanId=202569580851811194&start=1758653023818&end=1758656623818&paused=false)
using `ddtrace` without the fix (just OpenAI span, missing LangChain
span. Output does not directly include the `ImportError` but
`breakpoint`ing in does reveal it).

[Trace](https://dd.datad0g.com/llm/traces?query=%40ml_app%3Asam-test%20%40event_type%3Aspan%20%40parent_id%3Aundefined&agg_m=count&agg_m_source=base&agg_t=count&fromUser=false&sp=%5B%7B%22sp%22%3A%7B%22width%22%3A%22min%28100%25%2C%20max%28calc%28100%25%20-%20var%28--ui-page-left-offset%29%20-%2016px%29%2C%20900px%29%29%22%7D%2C%22p%22%3A%7B%22eventId%22%3A%22AwAAAZl4Gcv15UA_aQAAABhBWmw0R2N2MUFBQXlvaEhFVkI0Y0FBQUEAAAAkZjE5OTc4MTktZDc5MS00ZDQyLTlhMWMtZTk2OGM5NTUzODYzAAAAGQ%22%7D%2C%22i%22%3A%22llm-obs-panel%22%7D%5D&spanId=12717218430539325303&start=1758653023818&end=1758656623818&paused=false)
using local `ddtrace` with the fix (Includes the LangChain parent span).


## Risks

Importing `langchain` only/directly will no longer trigger patching.
With that being said, that hasn't been a valid way to use langchain for
some time now.

## Additional Notes

MLOB-3970
Fixes #14590

Author: sabrenner

.riot/requirements/10d6b85.txt

@@ -0,0 +1,103 @@
+#
+# This file is autogenerated by pip-compile with Python 3.10
+# by the following command:
+#
+#    pip-compile --no-annotate .riot/requirements/10d6b85.in
+#
+ai21==4.2.0
+aiohappyeyeballs==2.6.1
+aiohttp==3.12.15
+aiosignal==1.4.0
+annotated-types==0.7.0
+anthropic==0.68.0
+anyio==4.11.0
+async-timeout==4.0.3
+attrs==25.3.0
+boto3==1.40.39
+botocore==1.40.39
+certifi==2025.8.3
+charset-normalizer==3.4.3
+cohere==5.18.0
+coverage[toml]==7.10.7
+dataclasses-json==0.6.7
+distro==1.9.0
+docstring-parser==0.17.0
+exceptiongroup==1.3.0
+fastavro==1.12.0
+filelock==3.19.1
+frozenlist==1.7.0
+fsspec==2025.9.0
+greenlet==3.0.3
+h11==0.16.0
+hf-xet==1.1.10
+httpcore==1.0.9
+httpx==0.28.1
+httpx-sse==0.4.0
+huggingface-hub==0.35.1
+hypothesis==6.45.0
+idna==3.10
+iniconfig==2.1.0
+jiter==0.11.0
+jmespath==1.0.1
+jsonpatch==1.33
+jsonpointer==3.0.0
+langchain==0.3.27
+langchain-anthropic==0.3.20
+langchain-aws==0.2.33
+langchain-cohere==0.3.5
+langchain-community==0.3.29
+langchain-core==0.3.76
+langchain-experimental==0.3.4
+langchain-openai==0.3.33
+langchain-text-splitters==0.3.11
+langsmith==0.4.31
+marshmallow==3.26.1
+mock==5.2.0
+multidict==6.6.4
+mypy-extensions==1.1.0
+numexpr==2.8.5
+numpy==1.26.4
+openai==1.109.1
+opentracing==2.4.0
+orjson==3.11.3
+packaging==25.0
+pandas==2.3.2
+pluggy==1.6.0
+propcache==0.3.2
+psutil==7.1.0
+pydantic==2.11.9
+pydantic-core==2.33.2
+pydantic-settings==2.11.0
+pygments==2.19.2
+pytest==8.4.2
+pytest-asyncio==0.23.7
+pytest-cov==7.0.0
+pytest-mock==3.15.1
+pytest-randomly==3.10.1
+python-dateutil==2.9.0.post0
+python-dotenv==1.1.1
+pytz==2025.2
+pyyaml==6.0.2
+regex==2025.9.18
+requests==2.32.5
+requests-toolbelt==1.0.0
+respx==0.22.0
+s3transfer==0.14.0
+six==1.17.0
+sniffio==1.3.1
+sortedcontainers==2.4.0
+sqlalchemy==2.0.43
+tabulate==0.9.0
+tenacity==8.5.0
+tiktoken==0.11.0
+tokenizers==0.22.1
+tomli==2.2.1
+tqdm==4.67.1
+types-requests==2.32.4.20250913
+typing-extensions==4.15.0
+typing-inspect==0.9.0
+typing-inspection==0.4.1
+tzdata==2025.2
+urllib3==2.5.0
+yarl==1.20.1
+zstandard==0.25.0

.riot/requirements/11b0e12.txt

@@ -0,0 +1,97 @@
+#
+# This file is autogenerated by pip-compile with Python 3.12
+# by the following command:
+#
+#    pip-compile --no-annotate .riot/requirements/11b0e12.in
+#
+ai21==4.2.0
+aiohappyeyeballs==2.6.1
+aiohttp==3.12.15
+aiosignal==1.4.0
+annotated-types==0.7.0
+anthropic==0.68.0
+anyio==4.11.0
+attrs==25.3.0
+boto3==1.40.39
+botocore==1.40.39
+certifi==2025.8.3
+charset-normalizer==3.4.3
+cohere==5.18.0
+coverage[toml]==7.10.7
+dataclasses-json==0.6.7
+distro==1.9.0
+docstring-parser==0.17.0
+exceptiongroup==1.3.0
+fastavro==1.12.0
+filelock==3.19.1
+frozenlist==1.7.0
+fsspec==2025.9.0
+greenlet==3.0.3
+h11==0.16.0
+hf-xet==1.1.10
+httpcore==1.0.9
+httpx==0.28.1
+httpx-sse==0.4.0
+huggingface-hub==0.35.1
+hypothesis==6.45.0
+idna==3.10
+iniconfig==2.1.0
+jiter==0.11.0
+jmespath==1.0.1
+jsonpatch==1.33
+jsonpointer==3.0.0
+langchain==0.3.27
+langchain-anthropic==0.3.20
+langchain-aws==0.2.33
+langchain-cohere==0.4.6
+langchain-community==0.3.29
+langchain-core==0.3.76
+langchain-openai==0.3.33
+langchain-text-splitters==0.3.11
+langsmith==0.4.31
+marshmallow==3.26.1
+mock==5.2.0
+multidict==6.6.4
+mypy-extensions==1.1.0
+numexpr==2.8.5
+numpy==2.3.3
+openai==1.109.1
+opentracing==2.4.0
+orjson==3.11.3
+packaging==25.0
+pluggy==1.6.0
+propcache==0.3.2
+psutil==7.1.0
+pydantic==2.11.9
+pydantic-core==2.33.2
+pydantic-settings==2.11.0
+pygments==2.19.2
+pytest==8.4.2
+pytest-asyncio==0.23.7
+pytest-cov==7.0.0
+pytest-mock==3.15.1
+pytest-randomly==3.10.1
+python-dateutil==2.9.0.post0
+python-dotenv==1.1.1
+pyyaml==6.0.2
+regex==2025.9.18
+requests==2.32.5
+requests-toolbelt==1.0.0
+respx==0.22.0
+s3transfer==0.14.0
+six==1.17.0
+sniffio==1.3.1
+sortedcontainers==2.4.0
+sqlalchemy==2.0.43
+tenacity==8.5.0
+tiktoken==0.11.0
+tokenizers==0.22.1
+tqdm==4.67.1
+types-pyyaml==6.0.12.20250915
+types-requests==2.32.4.20250913
+typing-extensions==4.15.0
+typing-inspect==0.9.0
+typing-inspection==0.4.1
+urllib3==2.5.0
+yarl==1.20.1
+zstandard==0.25.0