fix(grok): hide empty cache creation column (#1605)

* fix(grok): hide empty cache creation column

Focused Grok reports now suppress the cache-creation column when every selected row reports zero, while retaining cache reads, totals, JSON fields, and unified report columns. The shared table renderer exposes this as an opt-in display option so future non-zero Grok records remain visible. Add focused regression coverage and document the source-specific table behavior.

* test(grok): harden cache column regression

Read the shared renderer default in the regression test so the test guards the public default configuration. Link the Grok guide to the JSON output documentation for the stable cacheCreationTokens schema.