fix: support middle-click paste in text inputs

Paste the Linux primary selection at the clicked caret in shared input
fields, including identity import. Preserve read-only and disabled fields
and add Shift+Insert for clipboard paste. Cover mouse and keyboard paste
with GPUI interaction tests and document the supported gestures.

Co-Authored-By: GPT-6 <noreply@openai.com>

crates/ui/Cargo.toml

@@ -14,3 +14,6 @@ serde.workspace = true
 smallvec.workspace = true
 
 uuid = "1.10"
+
+[dev-dependencies]
+gpui = { workspace = true, features = ["test-support"] }