[Staking] Async Election and Export

[Ank4n]

closes #10142

Context

Era rotation in pallet-staking-async coordinates validator set changes between Asset Hub and Relay Chain. Currently:

Happy Era Lifecycle (On Polkadot):

• Session 0-4: Idle
• Start of Session 5: Election starts for era N+1
• At some point in Session 5: Election completes → validators sent to RC
• End of Session 5: AH Client on RC queues validators to session pallet
• End of Session 6: Queued validator set gets activated. RC returns activation timestamp to AH, and AH rotates active era to N+1.

Issue with current approach

1. If election takes longer than a session: Active Era will become longer than 6 sessions.
2. If we allow two sessions for election, and election completes faster: Active Era will be shorter than 6 sessions.

These issues are currently unlikely, but with elastic scaling (multiple cores in AH), they become more probable.

Solution

1. Start the election as soon as Era N starts for Era N+1.
2. Buffer elected validators in RC Client pallet and send them after the start of Session 5.

Result

This ensures we send elected validators for the next era always at the right time. It also has two good side effects:

1. Election can run for longer and do less work per block than it is doing currently. We can split the work well across the whole lifecycle of the era instead of very compute heavy election session blocks.
2. Validator/nominator snapshot for the era election has a much cleaner boundary.

---

Note for Reviewers

• This test demonstrates the change from older configuration to new.

Open Questions

• Should we get rid of PlanningEraOffset completely and make election start asap (not configurable)

---

TODO

• Early exit signed validation (in separate PR)
• Two equivalent test with different configurations of PlanningEraOffset and ValidatorSetExportSession, demonstrating same outcome except election timings.
• Documentation

---

Update:
I have removed the new storage LastEraActivationSessionReportEndingIndex and instead rely only on session index from staking. I guess there is some tiny bit value in creating this new storage since:
– this pallet already knows everything it needs to know via session reports,
– keeping its logic lean and self-contained, without adding a dependency on the staking pallet.

But the pallets already talk via AHStakingInterface and so I don't think above reason holds.

[Ank4n]

/cmd fmt

[Ank4n]

/cmd prdoc --force

[paritytech-workflow-stopper]

All GitHub workflows were cancelled due to failure one of the required jobs.
Failed workflow url: https://github.com/paritytech/polkadot-sdk/actions/runs/19857945333
Failed job name: check-runtime-migration

[paritytech-release-backport-bot]

Created backport PR for unstable2507:

• [unstable2507] Backport #10311: [Staking] Async Election and Export #10526 with remaining conflicts!

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin backport-10311-to-unstable2507
git worktree add --checkout .worktree/backport-10311-to-unstable2507 backport-10311-to-unstable2507
cd .worktree/backport-10311-to-unstable2507
git reset --hard HEAD^
git cherry-pick -x 90b7cafd8cfba509cc0f6ea476acc1a8c9ad0c11
git push --force-with-lease

[paritytech-release-backport-bot]

Created backport PR for stable2509:

• [stable2509] Backport #10311 #10527 with remaining conflicts!

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin backport-10311-to-stable2509
git worktree add --checkout .worktree/backport-10311-to-stable2509 backport-10311-to-stable2509
cd .worktree/backport-10311-to-stable2509
git reset --hard HEAD^
git cherry-pick -x 90b7cafd8cfba509cc0f6ea476acc1a8c9ad0c11
git push --force-with-lease

[paritytech-release-backport-bot]

Created backport PR for stable2512:

• [stable2512] Backport #10311 #10528 with remaining conflicts!

Please cherry-pick the changes locally and resolve any conflicts.

git fetch origin backport-10311-to-stable2512
git worktree add --checkout .worktree/backport-10311-to-stable2512 backport-10311-to-stable2512
cd .worktree/backport-10311-to-stable2512
git reset --hard HEAD^
git cherry-pick -x 90b7cafd8cfba509cc0f6ea476acc1a8c9ad0c11
git push --force-with-lease