Exercise 3: Fixing the Mock Drift Two Ways — Possible Solution
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THE FIXES
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    (a) MockNotificationSenderFixed.send() accepts (email, subject, body)
        and expect_call() takes matching arguments.

    (b) UserRegistrationServiceFixed.register() now calls
        self.sender.send(email, "Welcome!", "Thanks for signing up!")
        - the new 3-argument call, matching what RealNotificationSenderV2
        actually requires.

RESULTS
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    Fixed mock-based test: expectation_met = True -> test PASSES

    Confirming it now genuinely matches the real dependency:
      Real dependency call: worked fine, no crash

Both the mock-based test and a direct call against the real,
previously-crashing RealNotificationSenderV2 now succeed, using the
identical register() call in both cases.

WHY THIS WORKS AS AN ANSWER
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This closes the exact gap the chapter found: previously, the mock and
the real dependency had silently drifted apart (the mock stuck on the
old 2-argument signature while the real class and the code calling it
moved to 3 arguments), and only the mock-based test's own result was
ever checked day to day. Updating both the mock AND the calling code
together - not just one or the other - restores the property a test
double is supposed to guarantee: that passing against the double
means the code would also work against the real thing. Fixing only
the mock without fixing register() (or vice versa) would have left the
same category of bug in place, just moved to a different mismatched
pair.