Base chat handler refactor for custom slash commands (jupyterlab#398)

* Adds attributes, starts adding to subclasses

* Consistent syntax

* Help for all handlers

* Fix slash ID error

* Iterate through entry points

* Fix typo in call to select()

* Moves config to magics, modifies extensions to attempt to load classes

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* Moves config to proper location, improves error logging

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* WIP: Updates per feedback, adds custom handler

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* Removes redundant code, style fixes

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* Removes unnecessary custom message

* Instantiates class

* Validates slash ID

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* Consistent arguments to chat handlers

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* Refactors to avoid intentionally unused params

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* Updates docs, removes custom handler from source and config

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* Renames process_message to match base class

* Adds needed parameter that had been deleted

* Joins lines in contributor doc

* Removes natural language routing type, which is not yet used

* Update docs/source/developers/index.md

Co-authored-by: Piyush Jain <piyushjain@duck.com>

* Update docs/source/developers/index.md

Co-authored-by: Piyush Jain <piyushjain@duck.com>

* Update docs/source/developers/index.md

Co-authored-by: Piyush Jain <piyushjain@duck.com>

* Revises per @3coins, avoids Latinism

* Removes Configurable, since we do not yet have configurable traits

* Uses Literal for validation

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Piyush Jain <piyushjain@duck.com>
(cherry picked from commit 64e6daf)

packages/jupyter-ai/jupyter_ai/chat_handlers/base.py

@@ -20,6 +20,9 @@
 from jupyter_ai_magics.providers import BaseProvider
 from langchain.pydantic_v1 import BaseModel
 
+# necessary to prevent circular import
+from pydantic import BaseModel
+
 if TYPE_CHECKING:
     from jupyter_ai.handlers import RootChatHandler