feat(sandbox): add LiteLLMCompaction capability sized via litellm

Summary

Adds agents.extensions.sandbox.LiteLLMCompaction, a drop-in subclass of agents.sandbox.capabilities.Compaction whose default policy is sized from litellm.get_model_info() rather than the built-in OpenAI-only context-window registry.

Why

The existing Compaction capability ships a private _MODEL_CONTEXT_WINDOWS registry that catalogues OpenAI families only. When a litellm-routed model (Anthropic, Bedrock, Vertex, custom proxy aliases, ...) is used:

• CompactionModelInfo.maybe_for_model(model) misses the lookup.
• Compaction.sampling_params therefore cannot pick a DynamicCompactionPolicy and falls back to a hard-coded 240k StaticCompactionPolicy.
• A 1M-context model and a 200k model end up being compacted at the same point, regardless of their actual input window.

What this PR adds

A new optional capability under the existing openai-agents[litellm] extra:

• LiteLLMCompaction.for_model(model, *, threshold=0.8) — resolves the cap via litellm.get_model_info() and builds a DynamicCompactionPolicy sized to the reported max_input_tokens. Falls back to a conservative 200k window with a WARNING when litellm has no entry for the model (brand-new beta IDs, custom proxy aliases, Bedrock embedding entries with no max_input_tokens), so the run still benefits from compaction instead of failing.
• LiteLLMCompaction.for_context_window(context_window, *, threshold=0.8) — builds the same policy from an explicitly-provided cap, for callers whose config layer already applies per-org ceilings or test overrides that should clamp the compaction threshold below the litellm-reported window.

The class is a pure pydantic subclass of Compaction, so process_context, sampling_params, the type: "compaction" serialization tag, and direct LiteLLMCompaction(policy=...) construction all behave identically to the parent.

Compatibility

Purely additive — no existing released APIs change. The module is gated behind the existing openai-agents[litellm] optional extra via raise_optional_dependency_error (same pattern as RedisSession, DaprSession, MongoDBSession), and is exported from agents.extensions.sandbox.__init__ only when the extra is installed.

Docs

• New API reference page at docs/ref/extensions/sandbox/litellm_compaction.md, registered under Extensions > Sandbox capabilities > LiteLLM compaction in mkdocs.yml.
• docs/sandbox/guide.md now calls out the OpenAI-only registry limitation right after the built-in capabilities table and points readers at LiteLLMCompaction for litellm-routed deployments.

Translated docs under docs/ja/, docs/ko/, and docs/zh/ are intentionally untouched (they are generated).

Usage

from agents.extensions.sandbox import LiteLLMCompaction

# Cap derived from litellm's registry.
capability = LiteLLMCompaction.for_model("anthropic/claude-3-5-sonnet")

# Cap derived from an externally-resolved value (per-org ceiling, test override, ...).
capability = LiteLLMCompaction.for_context_window(my_app_config.max_input_tokens)

Test plan

New tests at tests/extensions/sandbox/test_litellm_compaction.py (8 tests, all passing locally) cover:

• for_context_window builds the right DynamicCompactionPolicy with the default (0.8) and custom thresholds.
• sampling_params emits the correct compact_threshold end-to-end (context_window * threshold).
• for_model calls litellm.get_model_info and uses max_input_tokens.
• for_model falls back to the 200k default both when litellm raises and when max_input_tokens is None (Bedrock-embedding-style entries).
• Direct LiteLLMCompaction(policy=StaticCompactionPolicy(...)) construction still works (parent contract preserved).

make format, make lint, and mypy on the touched files all pass cleanly. The pre-existing mypy/pyright errors in function_schema.py, openai_responses.py, mount_lifecycle.py, and examples/tools/computer_use.py reproduce on main and are unrelated to this change.

Issue number

N/A

Checks

• I've added new tests
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass