Split large UID sets into bounded FETCH commands

[mpscholten]

Summary

• split large fetchByByteStringSet inputs across bounded UID FETCH commands
• keep generated wire lines at approximately 1,000 octets, following RFC 2683 section 3.2.1.5
• concatenate responses from every chunk while preserving command order
• add regression coverage at the exact command-line boundary

Context

Follow-up to #121, addressing the review feedback about unbounded UID sequence sets.

Validation

• Nix build with GHC 9.10.2
• imap-parsers: 55 cases, 0 errors, 0 failures
• Haddock generation