fix: persist preview and preview_rows fields after datastore push

[Copilot]

preview and preview_rows resource fields were being set via resource_update before datastore_create (with calculate_record_count=True), which internally modifies the resource and overwrites our fields.

Changes

• Reorder operations in both MetadataStage.process() and jobs_legacy.py: call send_resource_to_datastore() first, then _update_resource_metadata() / update_resource()
• Re-fetch resource via dsu.get_resource() before setting our fields, so we operate on the latest state post-datastore_create

# Before (fields get clobbered):
self._update_resource_metadata(context)      # sets preview, preview_rows
dsu.send_resource_to_datastore(...)           # datastore_create overwrites resource

# After (fields persist):
dsu.send_resource_to_datastore(...)           # datastore_create does its thing
self._update_resource_metadata(context)       # re-fetches resource, then sets fields

[jqnatividad]

Superseded by #292 (commit 20e835b). The Prefect 3 migration deleted jobs_legacy.py (the file this PR also patched), which is why this PR couldn't be cleanly rebased from the fork. #292 cherry-picks the still-applicable metadata.py half and credits the original work in the commit body. Thanks @copilot-swe-agent!