Derive the connection data source schema from the resource - #131

Stacked on #130, merge that first. Until #130 lands, the diff and commit list
below also contain its commit. Once it merges I'll rebase this branch onto main
and force-push, leaving a single commit here.

#130 fixes the panic from #129 at the point where it blows up. This PR removes the
condition that made it possible.

The data source hand-maintained a second copy of the resource's schema and the two
had drifted. That drift is what let the shared copier reach for password on a
schema that never declared it. The data source now derives its schema from
connectionSchema, password is named once in connectionLookupOmits, and a test
pins that the copier and the schema agree, so the next attribute added to the
resource can't silently go missing from the data source.

With value preservation in one place, this also fixes a second bug: the API returns
data_region and valid_until normalized, and a data_region even when none was
configured. Both are ForceNew and neither is Computed, so any API value written
to state recreates the connection on every apply. The configured values are now
always kept, not just when set. password is unchanged in effect; only create
returns it.

🤖 Generated with Claude Code