fix(cohere-embedding): add batch size validation with 96 limit for Cohere API

[yarikama]

Description

Add batch size validation for Cohere Embedding API to enforce the 96 batch size limit imposed by Cohere's API. This prevents runtime errors by catching invalid batch sizes during initialization, providing better error messages and preventing unnecessary API calls that would fail.

The change adds:

• MAX_EMBED_BATCH_SIZE constant set to 96
• Validation in init method to check embed_batch_size parameter
• Clear error message when batch size exceeds the limit
• Updated docstring to document the limitation
• Unit tests to verify the validation works correctly

Fixes # (issue)

New Package?

Did I fill in the tool.llamahub section in the pyproject.toml and provide a detailed README.md for my new integration or package?

• Yes
• No

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Your pull-request will likely not be merged unless it is covered by some form of impactful unit testing.

• I added new unit tests to cover this change
• I believe this change is already covered by existing unit tests

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added Google Colab support for the newly added notebooks.
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I ran uv run make format; uv run make lint to appease the lint gods