Fix react agent v2 take 4

This is a high priority MR for Switch to Chat Agent V2 (gitlab-org#13533). Please prioritize the review and merge.

What does this MR do and why?

We mistakenly reverted !165606 (merged) for fixing broken master. So we're merging again.

The changes are identical to Refactor ReAct Agent V2 (!165606 - merged). In addition, it includes a fix for the root cause Add selective disabled flag for v2 chat (!165504 - merged). See !165880 (f39b182c).

Closes Parse unformatted answers from LLM (#480563)

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