Improve docstrings + minor code refactor

[lhy-hoyin]

This PR includes:

• Adding, formatting and improving of docstring comments
• Updating absolute imports to relative imports (similar to other files)

---

[!IMPORTANT] Improved docstrings, switched to relative imports, and used a constant for EMBEDDING_SIZE for better code clarity and consistency.

• Docstrings:
  • Improved and formatted docstrings across multiple files for clarity and consistency.
  • Added detailed attribute descriptions in exception classes like AgentNotFoundError and UserNotFoundError.
• Imports:
  • Updated absolute imports to relative imports in files like truncation.py, worker.py, and messages.py for consistency.

^{This description was created by for c27aebf2cdfb27a2f94306001140974ad2bb3659. It will automatically update as commits are pushed.}

[creatorrr]

This change needs a fix @lhy-hoyin . See the lint-and-type-check github action. It's throwing an error:

  File "<frozen importlib._bootstrap_external>", line 995, in exec_module
  File "<frozen importlib._bootstrap>", line 488, in _call_with_frames_removed
  File "/home/runner/work/julep/julep/agents-api/tests/test_activities.py", line 10, in <module>
    from tests.fixtures import cozo_client, test_developer_id, test_doc
  File "/home/runner/work/julep/julep/agents-api/tests/fixtures.py", line 39, in <module>
    from tests.utils import patch_embed_acompletion as patch_embed_acompletion_ctx
  File "/home/runner/work/julep/julep/agents-api/tests/utils.py", line 12, in <module>
    from tests.fixtures import EMBEDDING_SIZE
ImportError: cannot import name 'EMBEDDING_SIZE' from partially initialized module 'tests.fixtures' (most likely due to a circular import) (/home/runner/work/julep/julep/agents-api/tests/fixtures.py)

[lhy-hoyin]

Hi @creatorrr , I have implemented a fix for the issue. Please assist to trigger GitHub Actions checks.