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/assistant_context/Cargo.toml

@@ -11,6 +11,9 @@ workspace = true
 [lib]
 path = "src/assistant_context.rs"
 
+[features]
+test-support = []
+
 [dependencies]
 agent_settings.workspace = true
 anyhow.workspace = true