Stronger preconditions before calling wait_for_commit - expect primary unanimity

[eddyashton]

See explanatory comment here: #7702 (comment)

Should resolve #7702 and #7687.

[Copilot]

Pull request overview

This PR strengthens test preconditions around elections by requiring primary unanimity before proceeding to commit synchronisation, addressing flaky “Multiple commit values” failures seen after leadership changes/suspensions.

Changes:

• Add a wait_for_primary_unanimity() barrier after force_become_primary() transitions, ensuring all nodes converge on the same primary.
• Replace wait_for_new_primary_in(...) with wait_for_primary_unanimity() in snapshot-selection tests before calling wait_for_node_commit_sync().

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/partitions_test.py	After forcing a new primary, wait for all nodes to agree on the primary before continuing.
tests/e2e_operations.py	After resuming suspended nodes, wait for primary unanimity before commit sync to reduce election/commit skew flakiness.