Stop generating accepted-line fingerprints (#38473)

## What changed

- Replace accepted-line fingerprint generation with diff-based added and deleted line counts.
- Keep `line_fingerprints` in the analytics payload as a statically empty field for schema compatibility.
- Remove the fingerprint types and public parsing API that are no longer used.

## Testing

- Update diff parsing tests to cover accepted-line counts without generating fingerprints.

GitOrigin-RevId: 4247c76c7b56b1a323f70f960823937352b84f7b

codex-rs/analytics/src/client_tests.rs

@@ -130,7 +130,7 @@ fn sample_accepted_line_fingerprint_event(thread_id: &str) -> TrackEventRequest
                 repo_hash: None,
                 accepted_added_lines: 1,
                 accepted_deleted_lines: 0,
-                line_fingerprints: Vec::new(),
+                line_fingerprints: [],
             },
         },
     ))