Suppress timestamp-only generated docs PRs

[danielporterda]

What changed

• Preserve the existing complete _generated metadata block when the published dashboard versions and repositories are unchanged.
• Require a non-empty material dashboard version summary before regenerating network-variable tabs or entering the PR creation/update path.
• Add regression coverage for timestamp/provenance-only source changes at both the generator and PR-orchestrator boundaries.

Why

The dashboard generator previously preserved generatedAt only when every other provenance field was identical, while the PR orchestrator treated any source-file diff as sufficient to proceed toward publication. Incidental collection metadata could therefore make a no-value update look substantive.

After this change, metadata-only collection runs return before rendered-tab generation and before create_or_update_pull_request is called. No timestamp-only PR is created. The existing stale-PR cleanup is only for removing a PR left open by the old behavior.

Validation

• direnv exec . python3 -m pytest tests/test_generate_network_component_versions.py tests/test_update_generated_reference_prs.py tests/test_summarize_version_changes.py -q (59 passed)
• direnv exec . ruff check scripts/generate_network_component_versions.py scripts/update_generated_reference_prs.py tests/test_generate_network_component_versions.py tests/test_update_generated_reference_prs.py
• git diff --check
• Live npm run generate:version-compatibility-dashboard plus npm run generate:network-variable-tabs produced no generated-output drift.
• python3 scripts/generate_all_reference_docs.py --dry-run listed all ten reference wrappers successfully.
• Manual Update generated docs run 30378676808 with targets=version-dashboard completed successfully, logged Source unchanged for Update generated docs; skipping generation, and created no version-dashboard/update PR against this branch.