fix(schema): preserve schema registry transaction invariants

[morluto]

Fixes #757 and #758.

Problem

A failed schema registration could leave an empty transaction-pending entry. Once that transaction ended, reconciliation called next() on the empty schema map and bricked the registry. Separately, a transaction could bind a second Pydantic model to an already committed schema URI because the conflict check ignored committed bindings.

Change

Create pending state only after descriptor registration succeeds, and keep committed registrations out of the transaction cache until reconciliation confirms commit. Reconciliation defensively discards an empty pending entry.

For model-backed schemas, calculate the deterministic descriptor URI and check both committed and transaction-pending bindings before descriptor registration. A conflicting model therefore cannot leave partial pending state.

Regression coverage

The schema-registry component suite now covers failed transactional descriptor registration, cross-transaction and intra-transaction model conflicts, same-model reattachment, and rollback followed by re-registration.

Validation

• make test-component TESTS=tests/component/schemas/test_schema_registry.py — 19 passed
• make check-static — passed

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Avoid using committed schemas as transaction witnesses

When a transaction re-registers a schema already present in _registrations, this branch ignores the committed cache and adds that existing URI to the pending set. If the same transaction then registers a new schema and rolls back, _reconcile_pending() sees the pre-existing URI as its witness and incorrectly promotes the entire pending set, including the rolled-back schema; subsequent registration returns the cached URI without recreating its missing descriptor. Preserve committed cache hits inside transactions, or track whether each witness was created by the transaction.

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