Expand tests to verify multipart structure and body content

[Faheem12005]

Resolves the TODO in testShouldSendEmailUsingUtf8ByDefault

this PR tightens and broadens multipart validation by enforcing expected MIME structure and body-part semantics across relevant tests.
testShouldSendEmailUsingUtf8ByDefault previously had a silent fallback for if the message wasn't a MimeMessage. I've replaced this with a stricter assertion so potentially broken email implementations don't pass through.
Defined a new helper assertMultipartSubtype to verify the multipart subtypes (eg: mixed, alternative) on both the outer message header and the parsed MimeMultipart object, and applied it to existing tests to increase multipart coverage.

Testing done

Modifications to tests only

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[ArpanC6]

@Faheem12005 The findFirstPartByContentType() helper looks good overall but one thing worth considering: the recursive call has no depth limit. For deeply or maliciously nested MimeMultipart structures this could theoretically lead to a stack overflow. Since this is test code it's low risk but adding a simple depth guard (e.g. maxDepth parameter) would make it more robust.