fix: abort committee rotation on session key registration failure

[Klapeyron]

Overview

register_committee_keys currently logs SessionInterface::set_keys failures and still hands the full committee to pallet_session. That pallet then drops members without NextKeys, so the live authority set diverges from CurrentCommittee/QueuedCommittee. AURA author index and BEEFY stake matching become undefined; a partial or empty set is a liveness/finality risk (#1895).

set_keys is treated as a black box. The session manager does not inspect or special-case the error (DuplicatedKey today, NoAccount, or a reason pallet_session adds later). Any Err aborts the hand-off.

Policy is all-or-nothing:

• Call set_keys for each unique member. First failure skips the whole incoming committee.
• new_session returns None, so pallet_session keeps the previous validators.
• The failed committee is not queued. QueuedCommittee stays the previous full set; the skipped epoch is stamped so rotation does not retry every block.
• A reduced committee is never installed.
• Genesis panics if the initial committee cannot be registered in full (empty default genesis config is still allowed).

A later epoch with a clean committee still rotates normally.

🗹 TODO before merging

• Ready

📌 Submission Checklist

• All commits are signed off (git commit -s) for the DCO
• Changes are backward-compatible (or flagged if breaking)
• Pull request description explains why the change is needed
• Self-reviewed the diff
• I have included a change file, or skipped for this reason:
• If the changes introduce a new feature, I have bumped the node minor version
• Update documentation (if relevant)
• Updated AGENTS.md if build commands, architecture, or workflows changed
• No new todos introduced

🧪 Testing Evidence

• Additional tests are provided (if possible)

cargo test -p pallet-session-validator-management --lib

Colliding session keys are only a fixture to make set_keys return Err. Tests cover: any registration failure skips the whole incoming set; all members rejected keeps the previous set; recovery on the next valid committee; genesis panics if the initial committee cannot register in full.

🔱 Fork Strategy

• Node Runtime Update
• Node Client Update
• Other:
• N/A

Runtime upgrade required so live chains pick up the new SessionManager behaviour.

Links

Closes #1895