Enable parallel backfill by eliminating shared state between providers

[kaxil]

Follow-up to #63269. The backfill command previously used shared providers.json state, meaning two breeze registry backfill runs for different providers couldn't safely execute concurrently.

This adds --provider and --providers-json flags to both extraction scripts (extract_parameters.py, extract_connections.py) so each backfill run uses an isolated temp providers.json and only scans the target provider. In --provider mode, modules.json is not written (it would be incomplete), so concurrent runs don't clobber each other.

What changed

• extract_parameters.py: --provider flag filters to single provider and skips modules.json/runtime_modules.json writes; --providers-json overrides the default search paths
• extract_connections.py: Same two flags — filters output to single provider and accepts a custom providers.json path
• registry_commands.py: Backfill command now creates a temp providers.json per version in a TemporaryDirectory, passes --provider/--providers-json to scripts, removes the _patch_providers_json save/restore pattern

Usage

Two terminal sessions can now safely backfill different providers simultaneously:

# Terminal 1
breeze registry backfill --provider amazon --version 9.15.0 --version 9.14.0

# Terminal 2
breeze registry backfill --provider google --version 14.0.0 --version 13.0.0

The registry-backfill.yml GitHub Actions workflow already uses a matrix strategy per provider, so this also makes individual CI jobs faster (no longer scanning 100+ providers per run).