chore: update AGENTS.md with Rust migration findings

[marc0olo]

Summary

• Clarifies that candid: in Rust icp.yaml is optional for backend-only examples; adds explanation of candid-extractor auto-extraction for the no-candid: case (icp-dev-env-rust:0.3.2+)
• Adds a "Management canister (Rust)" subsection documenting the ic-cdk-management-canister crate as the replacement for ic_cdk::api::management_canister (removed in ic-cdk 0.17+)
• Bumps the container version example in the Pending items section from 0.3.1 to 0.3.2
• Adds a Rust management canister checklist item to the dfx → icp-cli migration checklist

Test plan

• Review the diff to confirm all four changes are correctly applied and consistent with surrounding context
• Verify the ic-cdk-management-canister crate version 0.1.1 is accurate on crates.io

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