test: delegator migration all cases

[noa-starkware]

Note

Adds multiversion flow tests covering delegator balance changes and reward accrual during V1/V2→V3 migration, plus registers them in the mainnet test suite and updates flow ideas.

• Tests (flows.cairo):
  • Add multiversion delegator-rewards migration flows covering V1/V2→V3:
    • DelegatorRewardsMigrationIdxIsZeroFlow
    • DelegatorRewardsMigrationIdxLenIsOneFlow
    • DelegatorRewardsMigrationIdxIsOneFlow
    • DelegatorRewardsMigrationIdxIsLenFlow
    • DelegatorRewardsMigrationFirstRegularCaseFlow
    • DelegatorRewardsMigrationSecondRegularCaseFlow
• Test Suite (multi_version_tests.cairo):
  • Register new flows with MAINNET_LATEST tests invoking test_multi_version_flow_mainnet.
• Docs (flow_ideas.md):
  • Update ideas to enumerate new delegator migration/reward scenarios.

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