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

• [x] I've read the contributing guide
• [x] The relevant docs, if any, have been updated or created
• [x] 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.