bcr-pr-reviewer: Avoid repeating identical comments on PRs (#2803)

Previously, `postComment` only checked whether an identical comment had
been posted in the past 14 days. This caused informational notices (such
as sensitive metadata modification warnings, file limit alerts, and
maintainer notifications) to be re-posted every 2–4 weeks whenever PR
activity occurred.

These repeated comments generated notification noise and interfered with
GitHub's stale bot action by repeatedly resetting the PR's activity
timestamp, preventing inactive PRs from being automatically closed. See
https://github.com/bazelbuild/bazel-central-registry/pull/8523

This PR updates `postComment` to check all existing comments on the PR
so that identical bot comments are only posted once.