zeta2: Targeted retrieval search (#42240)

Since we removed the filtering step during context gathering, we want
the model to perform more targeted searches. This PR tweaks search tool
schema allowing the model to search within syntax nodes such as `impl`
blocks or methods.

This is what the query schema looks like now:

```rust
/// Search for relevant code by path, syntax hierarchy, and content.
#[derive(Debug, Clone, Serialize, Deserialize, JsonSchema)]
pub struct SearchToolQuery {
    /// 1. A glob pattern to match file paths in the codebase to search in.
    pub glob: String,
    /// 2. Regular expressions to match syntax nodes **by their first line** and hierarchy.
    ///
    /// Subsequent regexes match nodes within the full content of the nodes matched by the previous regexes.
    ///
    /// Example: Searching for a `User` class
    ///     ["class\s+User"]
    ///
    /// Example: Searching for a `get_full_name` method under a `User` class
    ///     ["class\s+User", "def\sget_full_name"]
    ///
    /// Skip this field to match on content alone.
    #[schemars(length(max = 3))]
    #[serde(default)]
    pub syntax_node: Vec<String>,
    /// 3. An optional regular expression to match the final content that should appear in the results.
    ///
    /// - Content will be matched within all lines of the matched syntax nodes.
    /// - If syntax node regexes are provided, this field can be skipped to include as much of the node itself as possible.
    /// - If no syntax node regexes are provided, the content will be matched within the entire file.
    pub content: Option<String>,
}
```

We'll need to keep refining this, but the core implementation is ready.

Release Notes:

- N/A

---------

Co-authored-by: Ben <ben@zed.dev>
Co-authored-by: Max <max@zed.dev>
Co-authored-by: Max Brunsfeld <maxbrunsfeld@gmail.com>

crates/language/src/buffer.rs

@@ -1618,6 +1618,18 @@ impl Buffer {
         self.parse_status.1.clone()
     }
 
+    /// Wait until the buffer is no longer parsing
+    pub fn parsing_idle(&self) -> impl Future<Output = ()> + use<> {
+        let mut parse_status = self.parse_status();
+        async move {
+            while *parse_status.borrow() != ParseStatus::Idle {
+                if parse_status.changed().await.is_err() {
+                    break;
+                }
+            }
+        }
+    }
+
     /// Assign to the buffer a set of diagnostics created by a given language server.
     pub fn update_diagnostics(
         &mut self,