feat(providers): make event buffer fixed (512) instead of configurable

Remove the EVENT_BUFFER_SIZE env var, the config field, the chart
eventBufferSize value, and the provider size-setter APIs. The per-provider
event buffer now always uses the built-in default capacity of 512; it is no
longer user-configurable, per request. Backpressure (no drops), metrics,
throttled saturation logging, and bounded parallel apply are unchanged.

Tests: drop the config/env override tests and the size-override tests;
the backpressure and concurrent-load tests now build a small buffer
in-package. Unit suite and -race pass.

Spec-Ref: helix-specs@d6808620:000408_investigate-and-fix-with