fix(conversations): model official input and output item unions

[leseb]

Summary

Closes #567.

• Replace opaque-object Conversation item contracts with the official OpenAI InputItem and ConversationItem unions using oneOf variants with type discriminators.
• Generate item schemas from the projected OpenAI specification via a new xtask command (sync-item-contracts), compiled at startup with jsonschema for runtime validation.
• Validate items at the owned API boundary: InputItem on create, ConversationItem on normalize/append-back, rejecting structurally invalid items with OpenAI-style error responses.
• Reconcile with aligned request contracts from fix(conversations): align create and update request contracts #625: add nullable_vec deserializer for null-as-empty items, remove dead nullable_initial_items_schema, fix integration test to include required OutputTextContent fields.

Test plan

• make lint passes (clippy, fmt, deps, docs, examples)
• make test passes (2534 unit + 457 integration + schema tests)
• Integration test append_back_persists_items_after_response fixed and passing
• Conformance report updated with refreshed drift counts

[praxis-bot]

PR Review

Summary: Reviewed all 18 changed files covering the InputItem/ConversationItem type split, generated JSON Schema validation at both API boundaries, the nullable_vec deserializer, LazyLock-based schema compilation, the oneOf-to-anyOf relaxation for runtime input validation, schema alignment across all item types (message, function_call, reasoning, program), the normalize_item -> validate_output_item pipeline, xtask tooling for schema generation and verification, and test coverage.

Overall: Clean. The design correctly separates input and output item types with schema-validated boundaries. The anyOf relaxation for InputItem allows overlapping EasyInputMessage and Item variants while ConversationItem retains strict oneOf. Normalization bridges the gap by adding id and status, and output validation confirms the result. Test coverage is thorough -- including the non_message_item_type_skips_normalization handler test that exercises function_call items through the full normalize-validate pipeline.

Severity	Count
Critical	0
Large	0
Medium	0

[praxis-bot]

We haven't heard back from you on this in some time, so we're going to mark it closed for now. However, if you're still interested in pushing this forward please don't hesitate to re-open and update 🖖

[franciscojavierarceo]

the current head still fails the required OpenAI conformance gate: it introduces six new POST /conversations request-drift fingerprints for the items schema, while ALLOW_NEW_FAILURES=false and no acknowledgement label is present. we need to align the generated contract with the reference or add the repository’s explicit acknowledgement, then rerun the failing integration fixture before this is mergeable.