DEPR: Treat tuple keys consistently in Resampler.__getitem__

[EsDeath-Boop]

Closes #66266.

Summary

Resampler.__getitem__ currently treats tuple keys as a list of column
labels. For example,

df.resample("h")["A", "B"]

behaves like

df.resample("h")[["A", "B"]]

instead of following DataFrame.__getitem__ semantics.

This PR updates Resampler.__getitem__ so that tuple keys are forwarded
to the underlying DataFrame selection. As a result:

• tuple keys correctly select MultiIndex columns
• tuple keys on non-MultiIndex columns raise KeyError

Tests

• Updated test_getitem to expect a KeyError for tuple keys on regular columns.
• Added a regression test for tuple-key selection on MultiIndex columns.