fix(sessions): reconcile pending input appends

[seanxuu]

Summary

Fix pending-input Session appends so they use the existing fail-closed reconciliation checkpoint. A retry appends once after an atomic failure, recognizes a committed lost-ack append without duplicating it, consumes only the checkpoint-owned pending-input prefix, and preserves already-completed guardrail results.

The RunState schema is bumped to 1.18 for the new pending-input ownership metadata. Checkpoint restoration validates input items before Session I/O, and Conversations reconciliation uses the same persistence normalization as its stored batch.

Test plan

• make typecheck
• uv run pytest -q tests/test_run_state_pending_input.py tests/test_run_impl_resume_paths.py tests/test_run_state.py tests/test_run_state_compatibility_corpus.py (710 passed)
• uv run ruff check src/agents/run_state.py src/agents/run_internal/session_persistence.py tests/test_run_state_pending_input.py
• The full verification wrapper reached 9,426 passing tests. The remaining test failures require git init --initial-branch=main, which this host's Git does not support.

Issue number

Closes #4775

Checks

• I've added new tests, if relevant
• I've run the repository verification wrapper
• I've confirmed applicable focused verification steps pass

[seratch]

The packaged compatibility assertion is now aligned, and both Linux packaged-contract checks pass. The runtime implementation and pending-input tests are unchanged, so the remaining requests still apply:

• Initialize a fresh OpenAIConversationsSession before reading its session ID.
• Account for Conversations API message normalization when comparing recovery fingerprints.
• Accept SDK-generated local-shell replay items during pending-input deserialization.
• Capture the streamed result's checkpoint after the expected exception, and add a controlled test proving that input appended during an in-flight session write survives recovery.

These are still needed before merging.

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[seanxuu]

Addressed all four requested follow-ups in commit 454d2215:

• Fresh OpenAIConversationsSession instances now initialize through the zero-item read path before their session ID is checkpointed.
• Lost-ack reconciliation canonicalizes Conversations API message defaults (including shorthand string content expanded to a single text part) while preserving required payload IDs such as item_reference.
• Pending Session-write checkpoint deserialization accepts SDK-generated local_shell_call_output replay items through the dedicated local-shell payload validator.
• Added a controlled streamed-failure regression test that appends input while the Session write is blocked, captures failed_result.to_state(), and verifies both inputs survive recovery exactly once.
  alidation: 189 focused tests passed; make typecheck passed (mypy and pyright).

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