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 patch version in a WebExtension manifest.json (e.g., 151.0.0 → 151.0.1)
• 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)

This is one part of a multi-repo change to automate the manual "bump minor version" step in the newtab XPI train-hop process.

Merge order

1. mozilla-releng/fxci-config — TC permissions must exist first
2. mozilla-services/cloudops-infra + mozilla-releng/k8s-autoscale — K8s workers
3. This PR — land on dev-landoscript branch first for e2e testing, then merge to master
4. mozilla-extensions/xpi-manifest — 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 → 150.0.1), 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)