PRS-104, updated clean accounts logic

[BoogVAr]

Summary of logic changes:

• Tightened cleanup criteria for zero-lamport accounts.
• Added support for purging accounts where:

  lamports are 0,
  data is empty,
  owner is the System Program.

• Preserved existing fork semantics where non-tombstone zero-lamport accounts (with meaningful owner/state) remain valid and are not removed.
• Updated the related cleanup test expectations to align with current zero-lamport policy.
• Added/updated regression coverage to confirm both scenarios:

  System-owned empty zero-lamport accounts are cleaned.
  Non-tombstone zero-lamport accounts are retained.