chore(docs): update Ruff rules section in AGENTS.md

Added details about frequently failing Ruff rules, including import sorting, type-checking guards, docstring spacing, and ternary suggestions. This update aims to improve code quality and adherence to the project's linting standards.

Author: kosiew

AGENTS.md

@@ -18,6 +18,11 @@ This repository contains Python bindings for Rust's DataFusion.
   - Python linting/formatting via Ruff
   - Rust formatting via `cargo fmt`
   - Rust linting via `cargo clippy`
+- Ruff rules that frequently fail in this repo:
+  - **Import sorting (`I001`)**: Keep import blocks sorted/grouped. Running `ruff check --select I --fix <files>` will repair order.
+  - **Type-checking guards (`TCH001`)**: Place imports that are only needed for typing (e.g., `AggregateUDF`, `ScalarUDF`, `TableFunction`, `WindowUDF`, `NullTreatment`, `DataFrame`) inside a `if TYPE_CHECKING:` block.
+  - **Docstring spacing (`D202`, `D205`)**: The summary line must be separated from the body with exactly one blank line, and there must be no blank line immediately after the closing triple quotes.
+  - **Ternary suggestions (`SIM108`)**: Prefer single-line ternary expressions when Ruff requests them over multi-line `if`/`else` assignments.
 
 ## Notes
 - The repository mixes Python and Rust; ensure changes build for both languages.