fix: use geodetic latitudes in haversine distance formula

[VibhorGautam]

Describe your change:

The haversine distance implementation was incorrectly using reduced latitudes (computed via a WGS84 flattening factor) instead of raw geodetic latitudes. Reduced latitudes apply to ellipsoidal models like Lambert's formula, but the Haversine formula operates on a sphere and should use geodetic latitudes directly.

What was wrong:

flattening = (AXIS_A - AXIS_B) / AXIS_A
phi_1 = atan((1 - flattening) * tan(radians(lat1)))
phi_2 = atan((1 - flattening) * tan(radians(lat2)))

This computes reduced latitudes, which account for Earth's ellipsoidal shape. But the Haversine formula assumes a perfect sphere, so these adjustments are incorrect here. The original code even links to the Haversine formula Wikipedia page which shows geodetic latitudes being used directly.

What this PR fixes:

• Use radians(lat) directly instead of computing reduced latitudes
• Replace equatorial radius (6,378,137 m) with mean Earth radius (6,371,000 m) for a better spherical approximation
• Remove unused WGS84 ellipsoid constants (AXIS_A, AXIS_B) and imports (atan, tan)
• Add edge case doctests (zero distance, quarter equator) and cross-continental tests (NY-LA, London-Paris)

References:

• https://en.wikipedia.org/wiki/Haversine_formula#Formulation
• https://github.com/mapado/haversine/blob/main/haversine/haversine.py#L183
• https://stackoverflow.com/a/4913653

Fixes #11308

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• An existing algorithm implementation is incorrect
• Documentation change?

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