Remove Profile.getSamlResponseXml() #424

Profile.getSamlResponseXml() hands the caller the response XML as received, without indicating which parts were
authenticated:

profile.getSamlResponseXml = () => samlResponseXml;

AGENTS.md's first invariant is "trust only the bytes that were signed", and this is a public
accessor that returns bytes that may not have been. When only the assertion is signed, nothing
around it is, and the accessor returns the same thing either way. Anything a caller decides on
the strength of what it returns may be a decision made on attacker-controlled input — the signature-wrapping
shape the rest of the library is built to prevent.

AGENTS.md names it as a known debt, alongside processValidlySignedAssertionAsync taking the
same XML as a parameter to back it.

Remove it in the next major. Callers should use getAssertionXml() or getAssertion(), which
are built from the verified content.

5.2.0 marks it @deprecated so it surfaces in the consumer's editor first.

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