Support assigned reviewers in LLM approval reviews

[guan404ming]

Why

• An LLM approval review was open to every user with the HITL permission.
• There was no way to route a decision to the people who actually own it.

How

• Add approval_assigned_users to name the reviewers allowed to answer.
• Forward those users to the review request, as HITLOperator already does.
• Validate the entry shape at parse time and reject the parameter on cores older than Airflow 3.1.

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Was generative AI tooling used to co-author this PR?

• Yes — Claude Code (Opus 5)

Generated-by: Claude Code (Opus 5) following the guidelines

[kaxil]

Round 1 is all addressed at 65c6efd, and I re-derived each thread against the code rather than going off the replies: assigned_users is passed unconditionally now, the 3.1.6 comparison boundary in the docs matches what hitl.py does at 3.1.5 versus 3.1.6, the stale-list caveat is still accurate against this branch's base, and the old-core test sits inside the old-core class so the sub-3.1 compat lane actually runs it.

One non-blocking doc thought, nothing gating:

llm.rst L283-285 gives FAB as the concrete answer for id, and FAB is not the default auth manager. Since a wrong id locks out every reviewer on 3.1.0 through 3.1.5, as the next sentence says, it would help to name the default too: [core] auth_manager defaults to SimpleAuthManager, whose get_id() returns the username, so there id is the username from simple_auth_manager_users.

[guan404ming]

Added in 6e25e2f: with the default SimpleAuthManager, id is the username from simple_auth_manager_users.