Add proposer re-org tests for non-default thresholds

[rahulbarmann]

Issue Addressed

• More reorg tests for re_org_threshold non-default values #9304

Proposed Changes

Add five proposer_boost_re_org_* tests that exercise non-default values of re_org_threshold, re_org_parent_threshold, and max_epochs_since_finalization. Each parameter gets a "permits" and a "blocks" case (where applicable) so that a test fails or changes behaviour if the parameter is silently ignored, which was the gap the issue identified.

Also adds a proposer_re_org_parent_threshold setter on BeaconChainBuilder to make the parent threshold drivable from ReOrgTest, mirroring the existing proposer_re_org_head_threshold setter.

Additional Info

Built against unstable ahead of #9177 landing, so the thresholds are still set via ChainConfig. If #9177 merges first, the builder calls in these tests can be swapped for the mut spec form per @michaelsproul's review comment.