Python: fix: inject synthetic tool results for pending frontend tool calls

[ranst91]

Description

When an agent calls a declaration-only (frontend/client-side) tool — such as
a generative UI component like pieChart — and the conversation ends without
a user message following, the tool call remains "pending" in the message
history with no corresponding tool result.

On the next request, OpenAI rejects the conversation with a 400 error:
"An assistant message with 'tool_calls' must be followed by tool messages
responding to each 'tool_call_id'."

_sanitize_tool_history already handles this when a user message arrives while
calls are pending — it injects synthetic results before the user turn. This
fix extends that logic to also cover the end-of-messages case, so pending
tool calls are always resolved before the history is sent to OpenAI.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

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[Copilot]

Pull request overview

Extends AG-UI message sanitization so that when a conversation ends with an assistant tool_calls message (common for declaration-only/frontend tools), synthetic tool results are injected to prevent OpenAI from rejecting the next request due to unmatched tool_call_ids.

Changes:

• Inject synthetic role="tool" function_result messages when the message list ends with pending tool calls.
• Add logging to indicate pending tool calls were resolved via synthetic results.

[Copilot]

Injecting a synthetic tool result at the end will cause any later real tool result for the same call_id (arriving in a subsequent request) to be treated as unmatched and dropped by _sanitize_tool_history (the synthetic result clears pending_tool_call_ids before the real tool message is processed). If frontend tools can legitimately return results asynchronously, consider limiting this end-of-history injection to tools known to be result-less (declaration-only), or adjust the sanitization logic to prefer/replace the synthetic placeholder when a real tool result is later received.

[Copilot]

This adds new behavior for the "assistant tool_calls at end of history" case, but there isn't a regression test covering it. Please add a unit test that passes [assistant(tool_call)] with no following user/tool message and asserts a synthetic role="tool" function_result is appended for the call_id.

[moonbox3]

Could you let us know, please, which version of agent-framework-ag-ui you're using?

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/ag-ui/agent_framework_ag_ui
_message_adapters.py	602	59	90%	102–103, 112–115, 118–122, 124–129, 132, 141–147, 187, 201, 205–207, 338, 428–431, 433–435, 482–484, 538, 541, 543, 546, 549, 565, 582, 604, 704, 720–721, 792, 814, 884, 919–920, 988, 1031
TOTAL	28017	3416	87%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
5454	20 💤	0 ❌	0 🔥	1m 29s ⏱️