fix(post-processing): keep the caller's index when parsing geography columns - #45086

SUMMARY

geohash_decode and geodetic_parse build their parsed frame as a bare DataFrame(), so it gets a fresh RangeIndex. _append_columns aligns on the index, so when the caller's frame is indexed by anything other than 0..n-1 nothing matches: the real rows get no coordinates and the parsed values arrive as extra rows.

Both now build the frame on the caller's index:

lonlat_df = DataFrame(index=df.index)

That makes the alignment a no-op instead of a mismatch. It is correct for either branch of _append_columns, the overwrite one and the append one, so it does not depend on #45083.

geohash_encode is left alone deliberately: it slices df[[latitude, longitude]], so it already inherits the caller's index. A test pins that it stays correct.

The index has never been carried here. Before #19116 _append_columns was a plain assign, which also aligns on the index, so this surfaced as silently null coordinates with the right row count; #19116's pd.concat(axis="columns") unions the index instead and turned that into extra rows as well. So this fixes a null-coordinate bug that is older than the row-count one, and both come from the same bare DataFrame().

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

N/A: backend only.

A pivot followed by a geodetic_parse, which is the shortest chain that hands these operations a non-RangeIndex frame:

pivoted = pivot(df=df, index=["region"], columns=[],
                aggregates={"geodetic": {"operator": "max"}})
geodetic_parse(df=pivoted, geodetic="geodetic", latitude="lat", longitude="lon")

before, 2 rows in and 4 out:

        geodetic   lat   lon
east  41.1 -73.5   NaN   NaN
west  40.7 -74.0   NaN   NaN
0            NaN  41.1 -73.5
1            NaN  40.7 -74.0

after:

        geodetic   lat   lon
east  41.1 -73.5  41.1 -73.5
west  40.7 -74.0  40.7 -74.0

A frame that already has a 0-based index is unchanged, which is why the existing tests pass untouched: they all build from lonlat_df, which has one.

TESTING INSTRUCTIONS

pytest tests/unit_tests/pandas_postprocessing/test_geography.py

Five cases are added, four of which fail on master:

The file goes from 5 tests to 10, and the five existing ones are unchanged. The post-processing suite is 191 passed, up from 186. tests/unit_tests/{pandas_postprocessing,charts,queries} run clean at 584 passed, 2 xfailed (both pre-existing). ruff check, ruff format --check and pre-commit run mypy are clean, and pylint --rcfile=.pylintrc rates the changed file 10.00/10.

I also ran this against #45083's branch, since both touch this area: 194 passed with the two changes together, so they compose.

ADDITIONAL INFORMATION

• Has associated issue: Fixes geohash_decode and geodetic_parse drop the caller's index, doubling the row count #45085
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API