feat: Add support for embeddings operations in langchain instrumentation

### What problem do you want to solve?

Currently, embeddings operations are not automatically instrumented by `loongsuite-instrumentation-langchain`.

### Describe the solution you'd like

- Add an instrumentation point for LangChain Embedding.
- Capture embeddings operations with [genai-util](https://github.com/alibaba/loongsuite-python-agent/tree/main/util/opentelemetry-util-genai).
- Add unit tests for instrumentations.

### Describe alternatives you've considered

_No response_

### Additional Context

_No response_

### Would you like to implement a fix?

None

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Start by locating the loongsuite-instrumentation-langchain package and its existing instrumentation unit tests, then read the opentelemetry-util-genai utility linked in the issue. Add coverage for LangChain Embedding operations and verify that embedding calls are captured through genai-util with the unit tests passing.

Written by the indexing model from the issue text.