BUG: PeriodIndex.from_fields rejects non-December quarterly freqs (#55784)

[karandhaodiyal28-hash]

• closes BUG: Unable to create PeriodIndex with freq of QuarterEnd with startingMonth != 12 #55784
• Tests added and passed
• All code checks passed
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file

_range_from_fields validated a quarterly freq with an exact-equality check
base != FreqGroup.FR_QTR.value. Quarterly period dtype codes are FR_QTR
plus the anchor-month offset (Q-DEC=2000, Q-JAN=2001, ..., Q-FEB=2002), so
the check only accepted Q-DEC and raised AssertionError: base must equal FR_QTR for every other quarterly anchor -- even though the equivalent scalar
Period(year=2014, quarter=3, freq=QuarterEnd(startingMonth=2)) works fine.

>>> import pandas as pd
>>> pd.Period(year=2014, quarter=3, freq=pd.offsets.QuarterEnd(startingMonth=2))
Period('2014Q3', 'Q-FEB')
>>> pd.PeriodIndex.from_fields(year=[2014], quarter=[3],
...                            freq=pd.offsets.QuarterEnd(startingMonth=2))
AssertionError: base must equal FR_QTR

The fix uses a frequency-group membership test instead of exact equality:

if FreqGroup.from_period_dtype_code(base) != FreqGroup.FR_QTR:

base itself is unchanged (it is still passed to libperiod.period_ordinal),
so the anchored quarterly frequency is preserved; only the guard is corrected to
accept any code in the FR_QTR group. Added a regression test parametrized over
all twelve quarterly anchors, asserting the result matches the scalar Period
path. Non-quarterly frequencies are still rejected, and the existing Q-DEC
behavior is unchanged.

[jbrockmendel]

is this reachable? if so, we should be raising something other than AssertionError

[jbrockmendel]

I suspect the scalar case is similarly broken if you want to address it in a followup