fix: recover agent message text from item/completed when deltas are missing

[maojindao55]

Problem

Some Responses API providers (e.g. Volcengine Ark / 火山方舟) emit reasoning items in output_item.added without the content/summary array fields that Codex's internal active-item state machine expects. When this happens, the state machine fails to register an active item, and all subsequent streaming deltas — including output_text.delta — are silently dropped with errors like:

ERROR codex_core::util: OutputTextDelta without active item

The model does generate text (confirmed by outputTokens in usage), and Codex successfully surfaces it via the item/completed notification. However, codex-acp ignored agentMessage items in completeItemEvent (return null), so the text was lost entirely. Users on affected providers only saw the reasoning/thinking content, with no actual assistant reply.

Root Cause (confirmed via logs)

Layer	TUI mode (codex CLI)	app-server mode (codex-acp)
stream_events_utils (SSE parsing)	✅ works	✅ works
codex_core::util (active-item state machine)	not used	❌ fails on providers with non-standard reasoning items

The TUI mode bypasses the state machine, so codex CLI works fine. The app-server mode uses it and drops deltas — but the full text is still available in item/completed.

Fix

Mirror the existing reasoning fallback pattern for agentMessage:

1. Track item IDs that received item/agentMessage/delta in a new seenAgentMessageDeltaItemIds set (in createTextEvent)
2. In completeItemEvent, if no deltas were seen for the agentMessage item, emit item.text as an agent_message_chunk
3. If deltas were seen, return null (no duplication)

This mirrors exactly how reasoning items are already handled (seenReasoningDeltaItemIds + createCompletedReasoningEvent).

Impact

• Providers with correct streaming (OpenAI, etc.): zero impact — seenAgentMessageDeltaItemIds contains the item ID, so the completed event returns null as before
• Providers with missing deltas (Volcengine Ark, potentially others): text is recovered from item/completed instead of being lost

Testing

Added agent-message-fallback.test.ts with two cases:

• "emits agent message text from item/completed when no deltas streamed" — verifies the fallback fires
• "does not duplicate agent message when deltas were streamed" — verifies no duplication for standard providers

All 280 existing tests still pass.

Reproduction

Provider config (Volcengine Ark):

model = "doubao-seed-2.0-lite"
model_provider = "volcengine-coding-plan"
model_supports_reasoning_summaries = true

[model_providers.volcengine-coding-plan]
base_url = "https://ark.cn-beijing.volces.com/api/coding/v3"
env_key = "ARK_API_KEY"
wire_api = "responses"

With this config, codex CLI (TUI) outputs text correctly, but any ACP client using codex-acp sees only thinking content and no assistant reply.