Remove stubs that leak states across SAML related tests

What does this MR do?

Remove stubs that leak states across tests

When running SAML-related feature and unit tests together (i.e. via fail-fast job in the CI), some stubs set up states that leak across other tests and causes unexpected failures.

This change replaces all instances of allow/expect_any_instance_of with allow/expect_next_instance_of. Additionally, the monkey-patched post method is also removed since it inadvertently overrides the post method in subsequent tests.

Relates to #329064 (closed)

Does this MR meet the acceptance criteria?

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