docs(conformance): pin the rate-limit block summary log line

What

Adds one "v1 decisions" bullet to conformance/README.md that pins a new rate-limit log line, rate_limit_block_summary. labkit-ruby adds the summary next, so the contract lands here first.

Why

Blocked requests in gitlab-rails are recorded as endpoint_id="unknown", so we can see that a rule blocked but not which endpoint. A Prometheus counter for this was rolled back for series volume. The summary counts blocks in memory and logs one line per (endpoint_id, rule) pair every interval instead. Context: gitlab-org/gitlab#638007.

What the bullet pins

• INFO, message=rate_limit_block_summary, endpoint_id, rate_limit_rule, integer block_count, optional feature_category. The key is the pair, and the first non-nil feature_category passed is the one logged.
• At most max_pairs pairs per process per interval (default 10). The rest fold into one other line.
• No line when nothing was counted.
• A logging failure reuses the fail-open WARN (message=rate_limit_error) and adds dropped_pairs.

Not in this MR

• No fields.yaml change. The existing rate-limit WARN already logs rule, name and identifier without entries there. Happy to add them if reviewers want them canonical.
• No scenario yet. The Ruby server pins gitlab-labkit ~> 5.6.0, which has no BlockSummary. The scenario follows with the pin bump after the labkit-ruby release.
• Go and Rust have no summary yet.

Checklist

• Teammate review
• Maintainer review