Bug 2025695 - landoscript: add extension_manifest_version_bump action

[JohanLorenzo]

Bug 2025695

Summary

• Adds a new extension_manifest_version_bump landoscript action that bumps the minor version in a WebExtension manifest.json (e.g., 151.0.0 → 151.1.0)
• Hooks into the new action via script.py dispatch and the task schema
• Refactors the action into smaller, independently testable functions (_validate_file, _fetch_file, _bump_manifest_version, _build_commit)

Merge order

None of these steps have happened yet. They must occur in this sequence:

1. mozilla-releng/fxci-config #912 — merge + run ci-admin to create TC clients
2. mozilla-services/cloudops-infra #6835 (dev) — merge + deploy
3. k8s-sops: add secrets for dev workers — manual step
4. mozilla-releng/k8s-autoscale #264 (dev) — merge to trigger dev worker deployment
5. This PR — land on dev-landoscript + e2e test
6. mozilla-services/cloudops-infra #6838 (prod) — merge + deploy
7. k8s-sops: add secrets for prod workers — manual step
8. mozilla-releng/k8s-autoscale #266 (prod) — merge to trigger prod worker deployment
9. This PR — merge to master
10. mozilla-extensions/xpi-manifest #271 — merge last

Verification

All verifications were done locally:

• Unit tests: 105 pass (uv run pytest in landoscript/)
• Local dry-run against staging Lando + real staging-firefox GitHub repo: action fetched manifest.json, generated correct diff (150.0.0 → 150.1.0), produced valid lando-actions.json
• Diff applied cleanly to a local bare git repo seeded with the real staging-firefox manifest.json
• Generated payload validated against landoscript_task_schema.json
• Lando API accepted the payload (HTTP 202)

[jcristau]

I don't love that this is an entirely new ad-hoc action. Couldn't we re-use the existing version_bump and tag actions?