Allow single-point Prometheus query_range requests

[milindsrivastava1997]

Bug

The Prometheus HTTP adapter rejects /api/v1/query_range when start == end with start must be before end. A range query over one evaluation timestamp should be accepted and return a one-sample matrix.

Reproduction

GET /api/v1/query_range?query=sum(metric)&start=1700000000&end=1700000000&step=60

Expected

HTTP success with a matrix evaluated once at 1700000000.

Actual

HTTP bad-data error: start must be before end.

Scope

This is an HTTP request-validation fix, independent of planning-snapshot window derivation.