Improve Mailable assertion error messages with expected vs actual values

[ahinkle]

Currently, when mailable assertions fail, developers only see generic error messages that don't show what was actually found vs what was expected. This forces developers to dig through code to understand why tests are failing.

Changes:

• assertHasSubject() - Shows expected vs actual subject
• assertFrom() - Shows expected vs actual sender information
• assertTo() / assertHasTo() - Shows expected vs actual recipients
• assertHasCc() - Shows expected vs actual CC recipients
• assertHasBcc() - Shows expected vs actual BCC recipients
• assertHasReplyTo() - Shows expected vs actual reply-to addresses
• assertHasTag() - Shows expected tag vs actual tags list
• assertHasMetadata() - Shows expected vs actual metadata values

Usage:

$mailable->assertHasSubject('Welcome to Laravel!');

// Before:
// "Did not see expected text [Welcome to Laravel!] in email subject."

// After:
// "Email subject does not match expected value.
// Expected: [Welcome to Laravel!]
// Actual: [Order Confirmation Email]"

$mailable->assertTo('john@example.com', 'John Doe');

// Before:
// "Did not see expected recipient [john@example.com (John Doe)] in email 'to' recipients."

// After:
// "Did not see expected recipient in email 'to' recipients.
// Expected: [john@example.com (John Doe)]
// Actual: [jane@example.com (Jane Smith)]"