[13.x] Add tests for Attachment::fromUrl() URL scheme validation

[mdalikadar]

Attachment::fromUrl() validates that the URL uses http or https scheme,
throwing an InvalidArgumentException for anything else. However, the Attachment
class had no dedicated unit test file.

This PR adds tests/Mail/AttachmentTest.php covering:

• http:// and https:// URLs are accepted
• ftp://, file://, mailto: and plain strings throw InvalidArgumentException
• ->as() and ->withMime() fluent methods work correctly
• Method chaining works correctly
• isEquivalent() returns true for same attachment, false for different paths
• fromData() creates the attachment with the correct filename