fix(sgl): guard zero-length lines, NaN fractions, and degenerate segments in linear referencing

[connerkup]

Summary

Fixes three interconnected edge-case bugs in the SGL linear referencing routines (sgl::linestring::interpolate, sgl::linestring::interpolate_points, and sgl::linestring::substring):

1. Infinite loop / thread hang in ST_LineInterpolatePoints:
   When invoked with repeat=true on a zero-length linestring (e.g. LINESTRING(5 5, 5 5)), actual_length evaluates to 0.0. In the multi-point accumulation loop, next_target is incremented by frac * actual_length = 0.0. The condition while (total_length + segment_length >= next_target) (0.0 >= 0.0) never terminates, causing continuous heap allocation and freezing the worker thread.
2. Division by zero / NaN coordinate corruption in ST_LineSubstring:
   When a linestring contains degenerate zero-length segments (e.g. leading identical vertices like LINESTRING(1 1, 1 1, 2 2) at start_fraction = 0.0 or a fully collapsed line LINESTRING(5 5, 5 5)), segment_length is 0.0. Evaluating sfrac = remaining / segment_length performs 0.0 / 0.0, injecting IEEE 754 NaN coordinates into the geometry (LINESTRING (nan nan, 1 1, 1.5 1.5)). This also results in inverted bounding box extents (min_x = +1.79e308, max_x = -1.79e308).
3. Division by zero in ST_LineInterpolatePoint on collapsed lines:
   On a zero-length linestring LINESTRING(5 5, 5 5), linestring::interpolate computed sfrac = 0.0 / 0.0 = NaN, returning POINT EMPTY instead of returning the coordinate POINT (5 5).
4. NaN input handling:
   Guarded beg_frac and end_frac against NaN inputs, preventing fall-through that previously emitted a silently corrupted vertex at (0, 0).

Changes

• src/sgl/sgl.cpp:
  • In linestring::interpolate: early return with vertex_array if actual_length == 0; skip zero-length segments (segment_length == 0); reject NaN fractions; fall back to terminal vertex on floating-point precision edge.
  • In linestring::interpolate_points: early return with POINT if actual_length == 0 or if (frac * actual_length) <= 0.0 (avoiding infinite loops on zero-length and subnormal underflow); skip zero-length segments; reject NaN fractions.
  • In linestring::substring: early return with vertex_array if total_length == 0; reject NaN fractions; safely clamp beg and end without division when encountering degenerate segments where segment_length == 0.
• src/sgl/sgl_test.cpp:
  • Added comprehensive unit tests and adversarial suites covering 2D/3D/4D (Z, M, ZM), degenerate lines, duplicate leading/middle vertices, NaN/Inf fractions, negative values, and underflow. Tested clean with AddressSanitizer and UndefinedBehaviorSanitizer (-fsanitize=address,undefined).
• test/sql/geometry/st_lineinterpolatepoint.test:
  • Added SQL regression tests for ST_LineSubstring, ST_LineInterpolatePoint, and ST_LineInterpolatePoints on degenerate lines and duplicate vertices.