[Docs] Enforce Black formatting in contributor docs

Black formatting is now enforced in CI. Update CLAUDE.md, AGENTS.md,
and CONTRIBUTING.md to instruct contributors and AI agents to run
`poetry run black .` before committing, and add VS Code setup guidance.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

AGENTS.md

@@ -254,7 +254,7 @@ See `CLAUDE.md` and the `Makefile` for standard commands. Key notes:
 - `openapi-core` must be installed (`poetry run pip install openapi-core`) for the OpenAPI compliance tests in `tests/test_litellm/interactions/`.
 - The `--timeout` pytest flag is NOT available; don't pass it.
 - Unit tests: `poetry run pytest tests/test_litellm/ -x -vv -n 4`
-- Black `--check` may report pre-existing formatting issues; this does not block test runs.
+- **Before committing, always run `poetry run black .` to format your code.** Black formatting is enforced in CI.
 - If `poetry install` fails with "pyproject.toml changed significantly since poetry.lock was last generated", run `poetry lock` first to regenerate the lock file.
 
 ### Lint

CLAUDE.md

@@ -20,6 +20,7 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 - `make format` - Apply Black code formatting
 - `make lint-ruff` - Run Ruff linting only
 - `make lint-mypy` - Run MyPy type checking only
+- **Before committing, always run `poetry run black .` to format your code.** Black formatting is enforced in CI.
 
 ### Single Test Files
 - `poetry run pytest tests/path/to/test_file.py -v` - Run specific test file

CONTRIBUTING.md

@@ -149,6 +149,19 @@ Apply formatting (auto-fixes issues):
 make format
 ```
 
+> **Black formatting is enforced in CI.** All PRs must pass the Black formatting check.
+>
+> - **AI coding agents** (Claude Code, Copilot, Cursor, etc.): `AGENTS.md` and `CLAUDE.md` instruct agents to run `poetry run black .` before committing.
+> - **VS Code users**: Install the [Black Formatter extension](https://marketplace.visualstudio.com/items?itemName=ms-python.black-formatter) and enable format-on-save:
+>   ```json
+>   {
+>     "[python]": {
+>       "editor.defaultFormatter": "ms-python.black-formatter",
+>       "editor.formatOnSave": true
+>     }
+>   }
+>   ```
+
 ### CI Compatibility
 
 To ensure your changes will pass CI, run the exact same checks locally: