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Problem

Provider errors such as No tool output found for function call call_119 are currently returned from session/prompt as an ACP internal error. That ends the interaction as a protocol failure instead of reporting a failed turn that the user can recover from.

Changes

• Report provider stream failures through an agent message and return end_turn, keeping the session available for another prompt.
• Mark unfinished tool calls as failed, notify the client, and persist their error results without changing completed tools.
• Finalize pending tool results on cancellation or stream failure so persisted history does not leave running tools behind.
• Keep genuine notification and persistence failures as errors.
• Reserve original Responses call IDs before renaming duplicates, preventing generated suffixes from colliding with IDs already in history.

Regression coverage

Tests cover failures before any tool runs and while a tool is pending, including client-visible notifications, persisted results, preservation of completed tools, and a successful next turn on the same connection. A serializer regression covers ID suffix collisions with out-of-order concurrent outputs.

Validation

On this branch against current upstream main:

• cargo test acp::service:: -- --test-threads=1 — 58 passed
• cargo test aisdk::providers::openai::tests:: -- --test-threads=1 — 53 passed
• cargo fmt --check
• git diff HEAD^ --check
• bash scripts/check-aisdk-boundary.sh

Also synced and pushed to yan-ad/crabcode:yan-flavor as b802f87; the flavor branch passed 72 ACP tests, 53 OpenAI tests, and all 8 ACP stdio tests. This PR contains only the fix, not the unrelated flavor changes.