agent: Enrich grep tool output with syntax information (#29601)

The `grep` tool used to include 4 lines of context around the match, but
the lines included would often be unhelpful. This PR improves this
behavior by using the range of the parent syntax node that contains the
full line(s) matched.

The match headers will also now include symbol breadcrumbs so that the
model can already gather code structure before/without reading files.

````md
### impl GitRepository for RealGitRepository › fn compare_checkpoints › L1278-1284
```rust
                let result = git
                    .run(&[
                        "diff-tree",
                        "--quiet",
                        &left.commit_sha.to_string(),
                        &right.commit_sha.to_string(),
                    ])
```
````

This positively impacts the `add_arg_to_trait_method` eval example with
better diff output, fewer tool failures, and reduced total turns.

Note: We have some plans to use a an "elision" approach where we would
combine all matches for a given file, skipping lines between them while
keeping symbol declaration lines. The theory is that this would be map
more closely to the expected input for edits. For now, this PR is a
significant improvement.

Release Notes:

- Agent: Enrich `grep` tool output with syntax information

crates/eval/src/example.rs

@@ -387,6 +387,7 @@ impl Response {
         cx.assert_some(result, format!("called `{}`", tool_name))
     }
 
+    #[allow(dead_code)]
     pub fn tool_uses(&self) -> impl Iterator<Item = &ToolUse> {
         self.messages.iter().flat_map(|msg| &msg.tool_use)
     }