fix: Add null checks for contentDisposition.parsed access

Fixed TypeError when parsing emails where contentDisposition.parsed
is undefined. This can occur with malformed emails or edge cases
where processHeaders() was not called on a node.

Added optional chaining to safely access:
- node.contentDisposition?.parsed?.value in isInlineTextNode()
- node.contentDisposition?.parsed?.value in isInlineMessageRfc822()
- node.contentDisposition?.parsed?.params?.filename in attachment handling
- node.contentDisposition?.parsed?.value in disposition assignment