docs: discourage #[async_trait] and #[allow(async_fn_in_trait)]

[bolinfest]

Why

We have run into two avoidable problems when introducing async trait APIs in Rust:

• #[async_trait] has caused materially worse build times in this repository.
• #[allow(async_fn_in_trait)] makes it too easy to ship a public trait without spelling out whether the returned future is Send, which hides an important part of the trait contract.

We already have a good example of the preferred alternative in #16630 / 3c7f013f9735, but that guidance currently lives only as prior art in the codebase. This PR documents the rule in AGENTS.md so contributors are more likely to follow the native RPITIT pattern before these two shortcuts spread further.

What Changed

• added Rust guidance in AGENTS.md discouraging both #[async_trait] and #[allow(async_fn_in_trait)]
• pointed contributors to the native RPITIT pattern with explicit Send bounds on the returned future
• clarified that implementations may still use async fn when they satisfy that trait contract

Verification

• docs-only change; no tests run

[chatgpt-codex-connector]

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