Allow max-age=0 value

[kpoelhekke]

Problem

• Setting max_age_header to 0 didn’t take effect.
• Reasons:
  1. WordPress treats taxonomy term name 0 as empty → value wasn’t saved.
  2. Header logic used truthy checks, so 0 was ignored when computing/setting headers (including batch min selection).

Fix

• Store zero using a sentinel term name zero so it persists; map back to integer 0 on read.

File

• wp/plugins/wpgraphql-smart-cache/src/Document/MaxAge.php