fix: use checked_add for total_fee_requested in process_delegation_cleanup

[amathxbt]

Summary

Fixes an unchecked arithmetic overflow in process_delegation_cleanup (src/processor/fast/undelegate.rs).

---

Bug — Plain + on commit_fee + SESSION_FEE_LAMPORTS can overflow

// Before
let commit_count = delegation_last_update_nonce.saturating_sub(1);
let commit_fee = COMMIT_FEE_LAMPORTS
    .checked_mul(commit_count)
    .ok_or(DlpError::Overflow)?;  // ← already protected
let total_fee_requested = commit_fee + SESSION_FEE_LAMPORTS;  // ← NOT protected

The per-commit fee multiplication is correctly protected with checked_mul, but the subsequent addition of the constant SESSION_FEE_LAMPORTS uses a plain + operator. For a long-lived delegation with a very large commit_count, commit_fee can approach u64::MAX. Adding SESSION_FEE_LAMPORTS on top then overflows, wrapping around to a small value. This causes the protocol to collect far less than the intended fee, letting the validator escape the session fee entirely.

Example: If COMMIT_FEE_LAMPORTS * commit_count = u64::MAX - 1 and SESSION_FEE_LAMPORTS = 5000, the result wraps to 4998 instead of the expected value, and total_fee_requested.min(total_lamports) silently charges the wrong amount.

---

Fix

// After
let total_fee_requested = commit_fee
    .checked_add(SESSION_FEE_LAMPORTS)
    .ok_or(DlpError::Overflow)?;

Consistent with every other arithmetic operation in this codebase.

---

Impact

• Severity: High — overflow in fee calculation causes under-collection of protocol/session fees
• Affected component: delegation-program / src/processor/fast/undelegate.rs

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• src/processor/fast/undelegate.rs

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