Don't unwrap a lambda whose callee contains a call

[Eljees]

Fixes #509.

The problem

NoRedundantLambda rewrites lambda: f() into f. That is only equivalent
while f is re-evaluated on every call. When the callee itself contains a
call, the autofix moves that inner call out of the lambda body, so it runs
once at definition time and its result is frozen:

# before
scheduler.add(lambda: datetime.now().isoformat())

# after the autofix
scheduler.add(datetime.now().isoformat)

The rewritten version binds isoformat to one fixed datetime instance
captured when the line was evaluated, instead of reading the clock on each
call. The lambda was not redundant, and the fix is applied automatically.

The change

Skip the report when the callee contains a call of its own:

if m.findall(call.func, m.Call()):
    return

Three valid cases pin the behaviour, covering the shapes this can take:
an attribute of a call result, a method on a returned object, and a
subscript of a call result.

Tests

	ran	result
main	389	OK
new cases, without the guard	392	FAILED (failures=3)
new cases, with the guard	392	OK

Without the guard the three new cases fail on the rule's own output rather
than on a count:

FAIL: test_VALID_10 (fixit.tests.NoRedundantLambda.test_VALID_10)
AssertionError: 1 != 0 : Expected zero reports for this "valid" test case.
Instead, found: LintViolation(rule_name='NoRedundantLambda', ...
message='The lambda that is wrapping datetime.now.isoformat is redundant.
It can unwrapped safely and used purely.')

No test that passes on main fails with the change. flake8 on the touched
file is clean.

One thing this deliberately does not cover

The existing invalid case

lambda x, y, z: (t + u).math_call(x, y, z)

is unsafe for a related but different reason: t + u is evaluated once
after unwrapping. That callee contains no call, so this change leaves it
reported exactly as before. I did not want to widen the scope without
knowing whether you consider that one intended — happy to extend the guard
to any non-trivial callee expression if you'd prefer.