feat(cli): add multiline input support with Alt+Enter

- Enable custom-bindings feature in rustyline
- Bind Alt+Enter to insert newlines in interactive and accumulative modes
- Update calculate_visual_lines() to handle embedded newlines correctly
- Add tests for multiline visual line calculation

Note: Shift+Enter is not distinguishable in standard terminals, so Alt+Enter
is used as the multiline input trigger.

crates/g3-cli/Cargo.toml

@@ -18,7 +18,7 @@ tracing-subscriber = { workspace = true, features = ["env-filter"] }
 serde = { workspace = true, features = ["derive"] }
 serde_json = { workspace = true }
 serde_yaml = "0.9"
-rustyline = { version = "17.0.1", features = ["derive", "with-dirs"] }
+rustyline = { version = "17.0.1", features = ["derive", "with-dirs", "custom-bindings"] }
 dirs = "5.0"
 tokio-util = "0.7"
 sha2 = "0.10"