Introduce DiagnosticsTool (#26670)

Release Notes:

- N/A

crates/assistant_tools/src/diagnostics_tool/description.md

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+Get errors and warnings for the project or a specific file.
+
+This tool can be invoked after a series of edits to determine if further edits are necessary, or if the user asks to fix errors or warnings in their codebase.
+
+When a path is provided, shows all diagnostics for that specific file.
+When no path is provided, shows a summary of error and warning counts for all files in the project.
+
+<example>
+To get diagnostics for a specific file:
+{
+    "path": "src/main.rs"
+}
+
+To get a project-wide diagnostic summary:
+{}
+</example>