feat(email): support inline PGP decryption for Outlook emails

[mqasimca]

Summary

Some email providers (notably Microsoft/Outlook) transform PGP/MIME encrypted messages when storing them. Instead of preserving the standard multipart/encrypted; protocol="application/pgp-encrypted" structure, they convert it to multipart/mixed with the PGP block as inline text.

This PR adds:

• Inline PGP detection: Fallback detection for -----BEGIN PGP MESSAGE----- blocks when the message is not in standard PGP/MIME format
• Better error propagation: GPG signing/encryption functions now return descriptive errors instead of generic "failed to build secure message"
• Comprehensive tests: Unit tests for the new extractInlinePGP function covering various scenarios

Problem

When using nylas email read <id> --decrypt on emails received via Microsoft/Outlook, decryption fails with:

Error: GPG decryption failed: message is not PGP/MIME encrypted (Content-Type: multipart/mixed; ...)

This happens because Outlook transforms the original PGP/MIME structure:

Original (Gmail preserves this):

Content-Type: multipart/encrypted; protocol="application/pgp-encrypted"

After Outlook transformation:

Content-Type: multipart/mixed; boundary="..."

--boundary
Content-Type: text/plain

-----BEGIN PGP MESSAGE-----
...encrypted content...
-----END PGP MESSAGE-----

Solution

The decryptGPGEmail function now:

1. First checks for standard PGP/MIME format (existing behavior)
2. If not PGP/MIME, searches for inline PGP blocks in the message body or MIME parts
3. Extracts and decrypts the inline PGP content

Changes

File	Changes
internal/cli/email/read_decrypt.go	Added extractInlinePGP() function, updated decryptGPGEmail() to use fallback
internal/cli/email/read_decrypt_test.go	Added 10 test cases for extractInlinePGP()
internal/cli/email/send_gpg.go	Changed build*Message() functions to return ([]byte, error) for proper error propagation

Test plan

• Unit tests pass (go test ./internal/cli/email/...)
• Linting passes (golangci-lint run)
• CI passes (make ci)
• Manual test: Decrypt PGP email from Gmail (PGP/MIME) ✓
• Manual test: Decrypt PGP email from Outlook (inline PGP) ✓

Compatibility

• Backward compatible: Standard PGP/MIME emails continue to work unchanged
• No breaking changes to CLI flags or API