fix: enable parsing of OpenAI-compatible agent mode messages

[shssoichiro]

Description

This implements proper parsing of agent mode messages for qwen3 and other OpenAI-compatible models which support tool use. This means that agent mode for Open-AI compatible models will now work properly, with the exception of messages which include multiple tool calls in a single message. Existing code does not support this, and this requires further changes of unknown scope.

Fixes #5419.

Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created
• The relevant tests, if any, have been updated or created

Screenshots

See issue #5419 for Before screenshots.

Tests

Added a new test suite for the sessionSlice reducer's streamUpdate function, including all cases related to this functionality.

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[github-actions]

All contributors have signed the CLA ✍️ ✅
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[shssoichiro]

I have read the CLA Document and I hereby sign the CLA

[recurseml]

😱 Found 1 issue. Time to roll up your sleeves! 😱

[DotDawson]

Are the PR test failures just flake? Reading through the logs, it looks like many of these tests don't even reach the point where extest run-tests is called. They seem to fail with 5xx errors during vscode extension installations. I don't see any reason this change would cause those failures.

The tests ran on June 12 @ 8:07 PM PST, which was not at the same time as the big google outage on June 12 between 1PM PST and 4PM PST.

I think the tests should be re-run.

[shssoichiro]

That was my thinking as well. I don't have permission to re-run the tests, so someone associated with the project will need to do it.