Fully support all mention kinds (#36134)

Feature parity with the agent1 @mention kinds:
- File
- Symbols
- Selections
- Threads
- Rules
- Fetch


Release Notes:

- N/A

---------

Co-authored-by: Cole Miller <cole@zed.dev>

crates/agent_ui/Cargo.toml

@@ -93,6 +93,7 @@ time.workspace = true
 time_format.workspace = true
 ui.workspace = true
 ui_input.workspace = true
+url.workspace = true
 urlencoding.workspace = true
 util.workspace = true
 uuid.workspace = true
@@ -102,6 +103,8 @@ workspace.workspace = true
 zed_actions.workspace = true
 
 [dev-dependencies]
+agent = { workspace = true, features = ["test-support"] }
+assistant_context = { workspace = true, features = ["test-support"] }
 assistant_tools.workspace = true
 buffer_diff = { workspace = true, features = ["test-support"] }
 editor = { workspace = true, features = ["test-support"] }