Update sink keying

Replace _DocFormatter with str.format for custom ID template rendering

_DocFormatter was silently producing empty entity_id renders in the
Alpine Python environment, causing every message to fall through to
auto-id. Plain str.format(**builtins) is simpler and guaranteed correct
for flat top-level field tokens.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>