Python: fix(ag-ui): preserve streamed text message id

[cyphercodes]

Motivation and Context

Fixes #6266.

Mixed AG-UI snapshots with both pending tool calls and streamed trailing text were assigning the streamed text message_id to the tool-call-only assistant message. The text message then received a generated ID, which broke reconciliation for the already-streamed text.

Description

When a snapshot contains both tool calls and accumulated text, this changes the tool-call-only assistant message to use a generated ID and preserves flow.message_id for the text assistant message.

A regression test covers the mixed tool-call/text snapshot and verifies that the text message keeps the streamed message ID while the tool-call message remains distinct.

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.

Validation:

• uv run --group dev pytest packages/ag-ui/tests/ag_ui/test_run.py::TestBuildMessagesSnapshot -q
• uv run --group dev poe syntax -P ag-ui
• uv run --group dev poe pyright -P ag-ui
• uv run --group dev poe test -P ag-ui

[Copilot]

Pull request overview

Note

Copilot was unable to run its full agentic suite in this review.

Updates message snapshot generation so that, when both tool calls and streamed text are present, the streamed text message keeps the original flow.message_id while the tool-call message gets a different ID.

Changes:

• Adjust _build_messages_snapshot message ID assignment to preserve streamed text message IDs in mixed tool-call/text snapshots.
• Add a regression test covering the mixed tool-call + text case to ensure IDs are preserved/isolated correctly.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
python/packages/ag-ui/agent_framework_ag_ui/_agent_run.py	Changes assistant/tool-call vs assistant/text message ID selection logic in snapshots.
python/packages/ag-ui/tests/ag_ui/test_run.py	Adds a test asserting streamed text message ID is preserved when tool calls are also present.

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