fix(agenticopenai): fall back to EasyInput for synthetic assistant text without item id/status

[hxxxi-malog]

What

assistantGenTextToInputItem now falls back to the plain EasyInput form ({"role":"assistant","content":"..."}) when the block carries neither an openai-item-id nor an openai-item-status extra, instead of always emitting an output message with empty id/status fields.

Why

Synthetic assistant messages (e.g. injected few-shot examples) carry no Responses API item metadata, so the current encoding sends "id":"" and status:"". Some OpenAI-compatible providers (e.g. BytePlus Ark) reject that with HTTP 400 InvalidParameter: ... unknown status: ., as reported in the issue. The reporter also verified that the same request succeeds once the item is sent in EasyInput form.

Explicit operation_id from endpoint metadata and messages converted back from real Responses API output (which always carry item id/status extras) keep the existing output-message encoding.

Fixes #977

How I tested

• New cases in TestAssistantGenTextToInputItem:
  • synthetic text without item id/status -> encoded as an EasyInput assistant message (no output message);
  • item id only (no status) -> still an output message, preserving current behavior for partial metadata.
• Ran go test ./... for components/model/agenticopenai:
  • targeted tests pass on Go 1.26.8 and Go 1.25.6 (the CI version);
  • note: on my Windows host, 33 mockey-based tests crash identically on pristine main and on this branch (a pre-existing local environment issue - CI runs them on Linux); all remaining tests pass.

[CLAassistant]

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