BUG/API: to_json with non-nano TimedeltaIndex

[jbrockmendel]

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Reproducible Example

# based on test_timedelta_as_label

df = pd.DataFrame([[1]], columns=[pd.Timedelta("1D").as_unit("us")])
result = df.to_json(date_format="iso")

expected = '{{"P1DT0H0M0S":{{"0":1}}}}'
assert result == expected  # nope! '{"P0DT0H1M26.400S":{"0":1}}'


result = df.to_json(date_format="epoch")
expected = '{{"86400000":{{"0":1}}}}'
assert result == expected  # nope! '{"86400":{"0":1}}'

Issue Description

#63196 affects test_timedelta_as_label so that we get "us" unit for df.columns (in the example i added the .as_unit("us"), which affects the to_json output. For the epoch format that might be plausible but the iso format case is pretty clearly wrong.

cc @WillAyd

Expected Behavior

N/A

Installed Versions

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