refactor(server): replace classify_uid Option<Option<u32>> with enum

[randomparity]

classify_uid took Option<Option<u32>> to encode three states (absent / present-size-unknown / present-size-known), carrying an #[expect(clippy::option_option)] and a doc admitting a custom enum would be clearer.

This introduces enum PreflightSize { Absent, PresentUnknown, Present(u32) }. preflight_sizes now builds a BTreeMap<u32, PreflightSize> holding only present states (PresentUnknown / Present); a lookup miss maps to Absent at the two call sites. classify_uid matches the three variants directly, dropping the nested Option and the suppression. The fetch_sizes trait and IMAP layer are untouched, per the issue.

Behavior is unchanged: the security-relevant NotFound/Oversize gating and the eligible-sum budget pre-check map state-for-state onto the prior encoding (verified by an adversarial review of every input case and the boundary at size == per_msg_cap).

Verification:

• cargo clippy -p rimap-server --all-targets --all-features clean (suppression removed, no option_option)
• 23 export_messages tests pass, including the six classify_uid_cases assertions remapped onto the enum
• cargo doc --no-deps --document-private-items shows zero export_messages.rs warnings

Closes #342

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