Expose BODYSTRUCTURE octet count as MessagePart.size (supersedes #184)

[odrobnik]

What

Carries #184 forward with the review finding fixed. The branch contains the original feature commit from @appinteractive as-is (6245c30), with main merged in on top plus a follow-up fix — so merging this PR (with a merge commit) makes GitHub mark #184 as merged automatically.

MessagePart gains an optional size property populated from the BODYSTRUCTURE body-fld-octets field, so clients can show size labels and enforce download caps from the structure fetch alone, without fetching part bytes:

/// The size of the part body in octets, as reported by BODYSTRUCTURE.
/// Zero is a valid size (an empty part); `nil` means the size is unknown
/// because the part was not built from a BODYSTRUCTURE response.
public let size: Int?

Fix over #184

The review on #184 flagged that the conversion collapsed a reported octet count of 0 to nil. In the pinned NIOIMAP 0.3.0 parser body-fld-octets is mandatory — a missing field makes the BODYSTRUCTURE invalid and never reaches this converter — so 0 is a legitimately reported size of an empty part, not a missing-value sentinel. The converter now passes the octet count through unchanged, keeping empty parts distinguishable from parts whose size metadata is unavailable (e.g. not built from BODYSTRUCTURE). The doc comment spells out these semantics, and the test asserting 0 → nil now asserts 0 is preserved.

Changes

• MessagePart: new size: Int? property, threaded through both public initializers and the Codable implementation (encoded with encodeIfPresent, so existing archives decode fine).
• MessagePart+BodyStructure: read fields.octetCount when flattening singleparts, passing it through as-is.
• MessagePartSizeTests: singlepart carries the octet count, zero is preserved, multipart children each carry their own size.

Note: the size is the encoded (transfer-encoding) octet count per RFC 3501 — for base64 parts roughly 4/3 of the decoded file size.

Test plan

• swift build clean
• Full suite: 352 tests in 48 suites pass locally on macOS

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