feat: add LiteLLM Rust workspace with Mistral OCR bridge

[ishaan-berri]

What

• Add minimal top-level litellm-rust/ workspace with only the crates needed for the first bridge: core, providers, and python-bridge.
• Shape Rust around LiteLLM route/provider responsibilities:
  • core/src/ocr/ owns OCR shared types and the OcrProviderConfig template trait.
  • providers/src/mistral/ocr/transformation.rs mirrors litellm/llms/mistral/ocr/transformation.py.
  • providers/src/ocr.rs owns Rust-side OCR provider/operation dispatch.
• Keep the PyO3 bridge intentionally thin: it exports one OCR function, ocr(payload), and delegates OCR routing to Rust.
• Add litellm/rust_bridge/ as the Python-side bridge package with an explicit OCR provider rollout enum and Python fallback behavior.
• Keep litellm/ocr/main.py small: after normal provider resolution, it asks litellm.rust_bridge.ocr for a Rust-backed config; disabled, unavailable, or unsupported Rust paths return the existing Python config unchanged.
• Add Rust CLAUDE.md/README rules for pure transforms, provider tree mapping, bridge shape, parity tests, and future host I/O boundaries.
• Add a path-scoped Rust GitHub Actions job that runs cargo fmt --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace on Rust changes.
• Add Python bridge tests for disabled, enabled, unavailable module, and unknown-provider fallback behavior.

Verification

• uv run black litellm/rust_bridge litellm/ocr/main.py tests/test_litellm/llms/mistral/ocr/test_mistral_ocr_rust_bridge.py
• uv run ruff check litellm/rust_bridge litellm/ocr/main.py tests/test_litellm/llms/mistral/ocr/test_mistral_ocr_rust_bridge.py
• uv run pytest tests/test_litellm/llms/mistral/ocr/test_mistral_ocr_transformation.py tests/test_litellm/llms/mistral/ocr/test_mistral_ocr_rust_bridge.py -q
• cd litellm-rust && cargo fmt --check
• cd litellm-rust && cargo clippy --workspace --all-targets -- -D warnings
• cd litellm-rust && cargo test --workspace

Scope

This PR keeps the rollout narrow: Mistral OCR only, route-level Python hook, Python fallback for every disabled/unavailable Rust path. Future OCR providers should add providers/src/<provider>/ocr/transformation.rs and register the provider in the Rust OCR dispatcher plus the Python rollout enum.

[codspeed-hq]

Merging this PR will not alter performance

✅ 16 untouched benchmarks

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_{Comparing codex/litellm-rust-mistral-ocr (ce27d80) with main (dcf1b44)}

[ishaan-berri]

Refiled from the CI-accepted branch name as #31033: #31033

[ishaan-berri]

Closing this codex-branch PR in favor of #31033 from litellm_hotfix_rust_mistral_ocr.