chore: remove bazel6, add bazel9 testing

[jbedard]

Changes are visible to end-users: no

Test plan

• Covered by existing test cases

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[aspect-workflows]

✨ Aspect Workflows Tasks

📅 Fri Jun 12 22:05:46 UTC 2026

✅ 5 successful tasks

• ✅ buildifier · ⏱ 23.3s · 🐙 GitHub Actions · ☑️ Check
  💬 Format complete (clean)
• ✅ format · ⏱ 26.2s · 🐙 GitHub Actions · ☑️ Check
  💬 Format complete (clean)
• ✅ gazelle · ⏱ 31.3s · 🐙 GitHub Actions · ☑️ Check
  💬 Gazelle complete (clean)
• ✅ test · ⏱ 25.2s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (3/3 passed)
• ✅ test (test-e2e-smoke) · ⏱ 27.2s · 🐙 GitHub Actions · ☑️ Check
  💬 Bazel test complete (1/1 passed)

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