fix: add GPT-5.5 aliases to sandbox compaction (#3039)

### Summary

Adds GPT-5.5 snapshot and pro aliases to the sandbox compaction model
context-window map so compaction threshold calculation recognizes them
as 1,047,576-token models.

### Test plan

- `uv run ruff check src/agents/sandbox/capabilities/compaction.py
tests/sandbox/test_compaction.py`
- `uv run pytest tests/sandbox/test_compaction.py`
- Verified `CompactionModelInfo.for_model()` locally for
`gpt-5.5-2026-04-23`, `gpt-5.5-pro`, and `gpt-5.5-pro-2026-04-23`
- Verified the model IDs exist via the OpenAI Models API; also ran a
live Responses API smoke with `gpt-5.5-2026-04-23`

### Issue number

N/A

### Checks

- [x] I've added new tests (if relevant)
- [ ] I've added/updated the relevant documentation
- [ ] I've run `make lint` and `make format`
- [x] I've made sure tests pass

Author: alfozan

src/agents/sandbox/capabilities/compaction.py

@@ -30,6 +30,9 @@ def _model_context_windows(models: tuple[str, ...], context_window: int) -> dict
             "gpt-5.4-pro",
             "gpt-5.4-pro-2026-03-05",
             "gpt-5.5",
+            "gpt-5.5-2026-04-23",
+            "gpt-5.5-pro",
+            "gpt-5.5-pro-2026-04-23",
             "gpt-4.1",
             "gpt-4.1-2025-04-14",
             "gpt-4.1-mini",

tests/sandbox/test_compaction.py

@@ -9,6 +9,9 @@
         ("gpt-5.4", 1_047_576),
         ("gpt-5.4-pro", 1_047_576),
         ("gpt-5.5", 1_047_576),
+        ("gpt-5.5-2026-04-23", 1_047_576),
+        ("gpt-5.5-pro", 1_047_576),
+        ("gpt-5.5-pro-2026-04-23", 1_047_576),
         ("gpt-5.3-codex", 400_000),
         ("gpt-5.4-mini", 400_000),
         ("gpt-4.1", 1_047_576),