Add Dependabot Bazel sync automation with manual backfill for existing PRs

[Copilot]

Dependabot PRs that update root pom.xml were failing Bazel sync checks because only Maven files were updated. This change adds automation to keep Bazel dependency artifacts aligned both for new Dependabot PR updates and for already-open Dependabot PRs.

• Per-PR Dependabot sync (existing/new updates)

  • Keeps dependabot-sync-bazel.yml focused on pull_request_target events for Dependabot PRs targeting master / master-java8 with root pom.xml changes.
  • Runs Bazel dependency regeneration and pushes updated MODULE.bazel / maven_install.json back to the Dependabot branch when needed.

• Manual backfill for open Dependabot PRs

  • Adds dependabot-sync-bazel-backfill.yml (workflow_dispatch) to process currently open Dependabot PRs in bulk.
  • Enumerates open PRs (with pagination), filters to Dependabot-owned branches and root pom.xml changes, then applies the same sync + repin flow and pushes updates per PR branch.

• Safety and scope controls

  • Restricts processing to same-repo Dependabot branches and supported base branches.
  • Uses a reusable temporary local branch during iteration to avoid branch accumulation while processing multiple PRs.

on:
  workflow_dispatch:
    inputs:
      backfill_open_prs:
        type: boolean
        default: false

[k8s-ci-robot]

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[brendandburns]

I don't think that we need Java or Bazel installed for this workflow, right?