feat: add attachment support to Resend adapter

[ChiragAgg5k]

Summary

• Remove the exception that blocked attachments in the Resend email adapter
• Add attachment handling: base64-encode file-based (getPath()) or string-based (getContent()) attachments and include them in the Resend API payload
• Validate total attachment size against MAX_ATTACHMENT_BYTES (25MB), consistent with the SMTP adapter

Test plan

• testSendEmailWithFileAttachment — sends email with a file-path-based attachment
• testSendEmailWithStringAttachment — sends email with raw string content attachment
• testSendEmailWithAttachmentExceedingMaxSize — verifies 25MB limit throws exception (passes locally)
• Existing Resend tests remain unaffected

[greptile-apps]

Greptile Summary

This PR enables attachment support in the Resend email adapter by removing the blocking exception and implementing proper attachment handling that is consistent with the existing SMTP adapter.

Key changes:

• Two-phase attachment processing: a first pass validates the total raw content size against MAX_ATTACHMENT_BYTES (25 MB), and a second pass builds the base64-encoded payload for the Resend API
• Explicit error guards for both filesize() and file_get_contents() failures (both return false on error), throwing descriptive exceptions instead of silently producing corrupt results
• The content_type field is included in each attachment object, letting the API/mail client use the declared MIME type rather than guessing from the filename
• New tests cover file-path attachments, raw string-content attachments, and the 25 MB limit enforcement

All three issues raised in the previous round of review have been addressed. The implementation aligns well with the SMTP adapter pattern and the Resend API specification.

Confidence Score: 5/5

Safe to merge — all prior P1 issues are resolved and no new issues were found.

All three previous P1 findings (silent file_get_contents failure, silent filesize failure, missing content_type) are explicitly addressed in this revision. The implementation is consistent with other adapters in the codebase (SMTP), the error paths are properly guarded, and the new tests provide solid coverage of the main scenarios. No remaining P0 or P1 issues.

No files require special attention.

Important Files Changed

Filename	Overview
src/Utopia/Messaging/Adapter/Email/Resend.php	Removes the attachment-blocking exception and adds proper attachment handling: size validation with explicit filesize()/file_get_contents() guards, base64 encoding, and content_type field — all previous P1 concerns have been addressed.
tests/Messaging/Adapter/Email/ResendTest.php	Replaces the old exception-throwing test with three new tests covering file attachments, string-content attachments, and the 25 MB size limit — test coverage is thorough and correct.

_{Reviews (2): Last reviewed commit: "fix: address PR review comments" | Re-trigger Greptile}