Investigate instructor client import errors (#1818)

fix: re-expose instructor.client for backward compatibility

## Describe your changes

This PR addresses the critical breaking change where `instructor.client`
was no longer directly accessible, causing `AttributeError` for users.

The changes re-expose `client` in `instructor/__init__.py` to restore
backward compatibility, allowing `instructor.client.Instructor` and
`instructor.client.AsyncInstructor` to function again. Deprecation
warnings are in place to guide users towards the new direct import
paths. Additionally, an internal reference in
`instructor/providers/writer/client.py` was updated for consistency.

## Issue ticket number and link

#1816

## Checklist before requesting a review

- [ ] I have performed a self-review of my code
- [ ] If it is a core feature, I have added thorough tests.
- [ ] If it is a core feature, I have added documentation.

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<!-- ELLIPSIS_HIDDEN -->

----

> [!IMPORTANT]
> Re-expose `instructor.client` for backward compatibility and update
internal references in `from_writer()`.
> 
>   - **Behavior**:
> - Re-expose `client` in `instructor/__init__.py` for backward
compatibility, allowing `instructor.client.Instructor` and
`instructor.client.AsyncInstructor` to work.
>     - Add deprecation warnings to guide users to new import paths.
>   - **Internal References**:
> - Update return type in `from_writer()` in
`instructor/providers/writer/client.py` to use `instructor.Instructor`
and `instructor.AsyncInstructor` directly.
> 
> <sup>This description was created by </sup>[<img alt="Ellipsis"
src="https://img.shields.io/badge/Ellipsis-blue?color=175173">](https://www.ellipsis.dev?ref=567-labs%2Finstructor&utm_source=github&utm_medium=referral)<sup>
for 91245da. You can
[customize](https://app.ellipsis.dev/567-labs/settings/summaries) this
summary. It will automatically update as commits are pushed.</sup>

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Author: jxnl

instructor/__init__.py

@@ -63,11 +63,16 @@
     "llm_validator",
     "openai_moderation",
     "hooks",
+    "client",  # Backward compatibility
     # Backward compatibility exports
     "handle_response_model",
     "handle_parallel_model",
 ]
 
+# Backward compatibility: Make instructor.client available as an attribute
+# This allows code like `instructor.client.Instructor` to work
+from . import client
+
 
 if importlib.util.find_spec("anthropic") is not None:
     from .providers.anthropic.client import from_anthropic

instructor/providers/writer/client.py

@@ -27,7 +27,7 @@ def from_writer(
     client: Writer | AsyncWriter,
     mode: instructor.Mode = instructor.Mode.WRITER_TOOLS,
     **kwargs: Any,
-) -> instructor.client.Instructor | instructor.client.AsyncInstructor:
+) -> instructor.Instructor | instructor.AsyncInstructor:
     valid_modes = {instructor.Mode.WRITER_TOOLS, instructor.Mode.WRITER_JSON}
 
     if mode not in valid_modes: