text-splitters, infra: fix libs/langchain/dev.Dockerfile so that th…

…e `text-splitter` directory is copied before poetry installation (langchain-ai#19214)

## Description
This PR modifies the settings in `libs/langchain/dev.Dockerfile` to
ensure that the `text-splitters` directory is copied before the poetry
installation process begins.

Without this modification, the `docker build` command fails for
`dev.Dockerfile`, preventing the setup of some development environments,
including `.devcontainer`.

## Bug Details

### Repro
Run the following command:

```bash
docker build -f libs/langchain/dev.Dockerfile .
```

### Current Behavior
The docker build command fails, raising the following error:

```
...
 => [langchain-dev-dependencies 4/5] COPY libs/community/ ../community/                                                                                0.4s
 => ERROR [langchain-dev-dependencies 5/5] RUN poetry install --no-interaction --no-ansi --with dev,test,docs                                          1.1s
------                                                                                                                                                      
 > [langchain-dev-dependencies 5/5] RUN poetry install --no-interaction --no-ansi --with dev,test,docs:
#13 0.970 
#13 0.970 Directory ../text-splitters does not exist
------
executor failed running [/bin/sh -c poetry install --no-interaction --no-ansi --with dev,test,docs]: exit code: 1
```

### Expected Behavior
The `docker build` command successfully completes without the poetry
error.

### Analysis
The error occurs because the `text-splitters` directory is not copied
into the build environment, unlike the other packages under the `libs`
directory. I suspect that the `COPY` setting was overlooked since
`text-splitters` was separated in a recent PR.

## Fix
Add the following lines to the `libs/langchain/dev.Dockerfile`:

```dockerfile
# Copy the text-splitters library for installation
COPY libs/text-splitters/ ../text-splitters/
```

libs/langchain/dev.Dockerfile

@@ -46,5 +46,8 @@ COPY libs/core ../core
 # Copy the community library for installation
 COPY libs/community/ ../community/
 
+# Copy the text-splitters library for installation
+COPY libs/text-splitters/ ../text-splitters/
+
 # Install the Poetry dependencies (this layer will be cached as long as the dependencies don't change)
 RUN poetry install --no-interaction --no-ansi --with dev,test,docs