fix(types): prevent panic in confirmation_time on empty attestations

[giwaov]

Summary

VerifiableLog::confirmation_time() indexed into the attestations slice using attestations[len / 2] without checking whether the slice was empty. When attestations.len() is 0, this evaluates to attestations[0] and panics with an index-out-of-bounds.

Both unit tests in types/src/ledger/log.rs construct a VerifiableLog with an empty attestations vec, so this is a realistic code path — not just a theoretical edge case.

Change

Return Option<Timestamp> instead of Timestamp, using slice::get so callers handle the absent case explicitly rather than the process crashing.

// Before
pub fn confirmation_time(&self) -> Timestamp {
    let num_attestations = self.pod_metadata.attestations.len();
    self.pod_metadata.attestations[num_attestations / 2].timestamp
}

// After
pub fn confirmation_time(&self) -> Option<Timestamp> {
    let attestations = &self.pod_metadata.attestations;
    attestations.get(attestations.len() / 2).map(|a| a.timestamp)
}

Test plan

• Confirm confirmation_time() returns None when called on a VerifiableLog with no attestations (as constructed in existing unit tests)
• Confirm all callers of confirmation_time() are updated to handle Option<Timestamp>