…kind discriminator

Phase 8 task #92 (D8.5-BE) — first-cut backend migration of the
non-agent chat path to the canonical ``UIMessage`` shape, scoped per
architect msg=01918929 + Weston msg=df87fe24 + earayu2 msg=f20d5034
hard-cut acceptance:

The inventory revealed the production "non-agent chat path" the
original D8.5 design assumed has already converged on the agent
runtime (``chat_completion_service.openai_chat_completions`` already
delegates to ``runtime_manager.turn_service.create_or_get_turn`` and
``ChatService.create_chat`` rejects non-AGENT bots). So the actual
#92 work is A+B+C only — adding the discriminator column for future
non-agent paths and migrating the user-visible chat history shape to
canonical UIMessage. The translator extension (``chat.text.delta`` /
``chat.completed``) and the ``StoredChatMessagePart`` /
``RedisChatMessageHistory`` deletion are deferred per architect /
Weston canonical lock.

Changes:

A. ``runtime_kind`` discriminator on ``agent_message`` table
- ``aperag/domains/agent_runtime/db/models.py``: new
  ``runtime_kind: str`` ORM column with values
  ``agent_runtime`` / ``direct_chat`` / ``rag_chat`` (mutually
  exclusive enum); existing rows backfill via
  ``server_default="agent_runtime"``. ``role`` keeps speaker
  semantics independent of the runtime that produced the message.
- ``aperag/migration/versions/...c8f2d34a51e7_add_agent_message_runtime_kind.py``:
  additive migration; downgrade drops the column.

B. ``ChatService._build_v3_chat_history`` rewrite
- Returns ``list[AgentTurnSnapshot]`` (one snapshot per assistant
  turn) instead of the legacy ``list[list[ChatMessage]]`` shape.
- Reuses ``snapshot_assembler.assemble_parts_from_artifacts`` (the
  #90 D8.4d projection) so historical turns expose the same
  ``UIMessagePart`` shape the FE consumes from the live SSE stream
  (D8 §2 wire/at-rest byte-equal).
- ``error_text`` for FAILED / CANCELLED turns surfaces an
  ``error_summary`` artifact's message, falling back to
  ``turn.error_message`` — mirrors the snapshot endpoint contract.
- The turn's user query lives at ``input_text`` on the snapshot
  envelope (rather than as a separate ``role=human`` ChatMessage)
  so the FE renders user/assistant from a single object per turn.
- Legacy ``_extract_artifact_text`` / ``_extract_references`` /
  ``_map_reference_item`` / ``_artifact_type_value`` /
  ``_coerce_timestamp`` helpers are retired alongside the legacy
  shape.

C. ``ChatDetails.history`` schema
- ``aperag/domains/conversation/schemas.py``: ``history`` is now
  ``Optional[list[AgentTurnSnapshot]]`` with explicit description
  citing D8 §2 byte-equal canonical and the new shape.
- The ``conversation.schemas`` ↔ ``agent_runtime.uimessage``
  ↔ ``agent_runtime.schemas`` ↔ ``conversation.schemas`` cycle is
  broken via ``TYPE_CHECKING`` import + a module-level
  ``ChatDetails.model_rebuild()`` hook at the bottom of
  ``conversation/schemas.py``. Pydantic resolves the forward ref at
  load time so the OpenAPI schema is fully populated.
- ``aperag/domains/agent_runtime/uimessage.py``: ``AgentTurnSnapshot``
  gains ``runtime_kind: RuntimeKind`` (default ``"agent_runtime"``)
  and ``input_text: Optional[str]`` so historical turns can render
  the user query without a separate envelope round-trip.
- ``TurnService.get_turn_snapshot`` writes both new fields on the
  live snapshot endpoint so live and historical reload paths match.

D. (deferred) Translator extension for ``chat.text.delta`` /
``chat.completed`` and ``StoredChatMessagePart`` /
``RedisChatMessageHistory`` deletion stay out of #92 per
Weston msg=df87fe24 / PM msg=01918929. The non-agent live path the
extension would have served does not exist in the current
codebase; reintroducing it is a feature task, not a refactor.

Tests:
- ``tests/unit_test/chat/test_chat_service.py`` rewritten:
  * ``test_get_chat_returns_canonical_uimessage_history`` pins the
    new shape (snapshot per turn with text + source-url +
    data-citation parts, runtime_kind, input_text)
  * ``test_get_chat_history_surfaces_error_text_for_failed_turn``
    pins the error_text contract for FAILED turns
  * ``test_get_chat_history_does_not_expose_legacy_chatmessage_shape``
    regression-guard against revert to ``list[list[ChatMessage]]``
- ``tests/unit_test/agent_runtime/test_agent_runtime_v3.py`` updated
  to import ``AgentTurnSnapshot`` from ``agent_runtime.uimessage``
  (the back-compat re-export through ``agent_runtime.schemas`` was
  retired to break the new cycle).

Per D10 §G hard gate 1 (comprehensive grep sweep) ran across
``aperag/`` + ``tests/unit_test/`` + ``tests/e2e_http/hurl/`` +
``tests/e2e_http/scripts/``: only the FE
``web/src/components/chat/chat-messages.tsx`` reads ``chat.history``
in the old shape — that is the explicit hand-off seam for #93
huangheng (per architect msg=6e53a7c4).

Gates: full unit suite 833 / 29 skip / 0 fail; ruff check + format
clean.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>