Handle nested literals in IMAP FETCH values

[mpscholten]

Summary

• stream byte-counted IMAP literals nested inside parenthesized FETCH values such as BODYSTRUCTURE
• preserve literal payload bytes and continuation-line separators in the returned value
• keep parentheses and quotes inside literal payloads from affecting the surrounding parser state

Without this, valid BODYSTRUCTURE responses containing literal MIME parameters fail with cannot parse FETCH value.

Test plan

• cabal test all --offline
• 50/50 IMAP parser tests pass
• regression coverage includes a literal filename, continuation-line delimiter preservation, and literal payload containing parser-significant parentheses and quotes

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