refactor gossip test executor

[gfukushima]

refactor some of the setup form the gossip test executors and fixed some nits from PR #11168

PR Description

Fixed Issue(s)

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

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Note

Medium Risk
Mostly test refactoring; the isPossibleDependentRoot change alters production gossip validation for proposer preferences and should be verified against reference tests.

Overview
Introduces GossipTestContext to centralize gossip reference-test setup (in-memory store, anchor, fork choice, execution-layer stub) and shared helpers: assertValidationResult for valid/reject/ignore expectations and a single FinalizedCheckpoint YAML type. Block-, attestation-, aggregate-, blob-, sync-committee, ePBS, and related executors now call GossipTestContext.create and drop duplicated boilerplate and per-file FinalizedCheckpoint classes.

Separately, GossipValidationHelper.isPossibleDependentRoot now returns false when the candidate block’s slot is already at or after the epoch start slot, matching spec is_valid_dependent_root (relevant to proposer-preferences gossip validation).

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[rolfyone]

LGTM