docs(litellm-rust): require the official Rust Style Guide in agent rules (#33867)

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: Ishaan Jaffer <155045088+ishaan-berri@users.noreply.github.com>

litellm-rust/AGENTS.md

@@ -15,3 +15,12 @@ Dependency direction (acyclic): litellm-core ← litellm-ai-gateway ← litellm-
 Adding a crate: default to a MODULE. New crate ONLY on a real trigger — separate artifact (binary/cdylib), proc-macro, shared foundation, or publishable standalone. A new provider or route is none of these.
 
 Adding a crate fails crates/core/tests/workspace_crate_allowlist.rs until you update its allowlist and this file — intentional.
+
+## Style
+
+All Rust in `litellm-rust/` follows the official Rust Style Guide:
+https://doc.rust-lang.org/style-guide/
+
+`rustfmt` implements its formatting by default, so run `cargo fmt` before committing; CI gates every PR on `cargo fmt --check`. Do not hand-format against rustfmt or add a `rustfmt.toml` that diverges from the default style.
+
+Beyond formatting, follow the guide's naming and idiom conventions rustfmt cannot auto-apply: `snake_case` items/functions/modules, `UpperCamelCase` types/traits/variants, `SCREAMING_SNAKE_CASE` constants/statics (acronyms as one word, e.g. `HttpClient`), and the import grouping and item ordering it prescribes. See CLAUDE.md for the detailed version.

litellm-rust/CLAUDE.md

@@ -77,6 +77,26 @@ such as `ai-gateway`, router hosts, or standalone servers:
 - Avoid `expect`/`unwrap` in server startup and request paths unless the panic is
   impossible by construction and documented.
 
+## Rust Style Guide
+
+All Rust in `litellm-rust/` follows the official Rust Style Guide:
+https://doc.rust-lang.org/style-guide/
+
+`rustfmt` implements the guide's formatting rules by default, so the mechanical
+side is enforced for you: run `cargo fmt` before committing and CI gates every
+PR on `cargo fmt --check` (see Checks). Do not hand-format against rustfmt or add
+a `rustfmt.toml` that diverges from the default style; the default style *is* the
+guide.
+
+The guide also covers conventions rustfmt cannot auto-apply; follow these too:
+- Naming: `snake_case` for items, functions, and modules; `UpperCamelCase` for
+  types, traits, and enum variants; `SCREAMING_SNAKE_CASE` for constants and
+  statics; acronyms count as one word (`HttpClient`, not `HTTPClient`).
+- Ordering and grouping the guide prescribes: imports grouped std / external /
+  crate-local, derives before other attributes, and consistent item order.
+- Idioms the guide recommends over the formatter fighting you (e.g. prefer
+  restructuring an over-long expression rather than forcing an awkward wrap).
+
 ## Constants
 
 Magic numbers and fixed strings go in a crate-level `constants.rs`, never