Add ContentTransferEncoding field to force CTE on a Part

[jhillyerd]

Allows callers to override the automatic Content-Transfer-Encoding
selection on a per-part basis. Valid values are "7bit", "8bit",
"base64", and "quoted-printable". Empty or unrecognised values
fall back to automatic detection.

This is useful for ASCII-safe content like PGP armor
(application/pgp-encrypted, application/pgp-signature) that would
otherwise be unnecessarily base64-encoded.

The forced CTE takes precedence over both the TextContent() gate
and the Encoder.ForceQuotedPrintableCte option.

Closes #395

[coveralls]

coverage: 86.563% (+0.1%) from 86.467% — cte-override into main

[Copilot]

Pull request overview

Adds a per-Part override for Content-Transfer-Encoding so callers can explicitly force "7bit", "8bit", "base64", or "quoted-printable" instead of relying on automatic detection—primarily to avoid unnecessary base64 encoding for ASCII-safe non-text payloads (e.g., PGP armor).

Changes:

• Introduces Part.ContentTransferEncoding plus a WithContentTransferEncoding(...) builder.
• Updates encoding logic to apply a forced CTE and (when needed) switch away from the streaming base64 path by materializing ContentReader content.
• Adds unit tests covering forced CTE behavior, fallback behavior, and precedence over encoder-level options.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
part.go	Adds the ContentTransferEncoding field, clones it, and provides a builder method.
encode.go	Applies the forced CTE during encoding and introduces resolveForcedCTE() mapping.
encode_test.go	Adds tests validating forced CTE behavior and precedence rules.

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