chore(tests): normalize recording IDs and timestamps to reduce git diff noise (#3676)

IDs are now deterministic hashes based on request content, and
timestamps are normalized to constants, eliminating spurious changes
when re-recording tests.

## Changes
- Updated `inference_recorder.py` to normalize IDs and timestamps during
recording
- Added `scripts/normalize_recordings.py` utility to re-normalize
existing recordings
- Created documentation in `tests/integration/recordings/README.md`
- Normalized 350 existing recording files

tests/integration/recordings/responses/ffd7b58fded8.json

@@ -1053,7 +1053,7 @@
           "prompt_tokens": 5,
           "total_tokens": 5
         },
-        "id": "f6acf878-03d6-4245-a55e-e01b68ddd8c8"
+        "id": "rec-ffd7b58fded8"
       }
     },
     "is_streaming": false