Fix repeated attachment content reads

[stevebauman]

Description

Fixes #166.

Calling Attachment::contents() currently drains the underlying PSR-7 stream because getContents() reads from the stream's current cursor position to the end. If a developer reads an attachment's contents first and then calls save(), the stream cursor is already at EOF, so the saved file is created with zero bytes.

This updates Attachment::contents() to rewind seekable streams before reading, so the high-level attachment API consistently returns the full attachment contents. Developers who need raw cursor-based stream behavior can still access the stream directly through contentStream().

Tests

• vendor/bin/pest tests/Unit/AttachmentTest.php
• vendor/bin/pest tests/Unit