Add vtctld get-vschema command

[planetscale-ai-tools]

Adds pscale branch vtctld get-vschema <database> <branch> --keyspace <keyspace> for reading live VSchema state through the API. The client and command preserve the existing raw JSON output conventions used by other vtctld commands.

Testing:

• go test ./internal/planetscale ./internal/cmd/branch/vtctld
• go vet ./internal/planetscale ./internal/cmd/branch/vtctld
• staticcheck ./internal/planetscale ./internal/cmd/branch/vtctld

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Security impact:

None. This is a client for a read-only, server-authorized API endpoint.

[planetscale-ai-tools]

Closing: the Inventory2 runbook GetVSchema / multi_tenant_spec pre-check is already covered by existing pscale keyspace vschema show (and update).

Verified on dev-aws-us-east-1-3 with a sharded keyspace: after pscale keyspace vschema update with multi_tenant_spec, pscale keyspace vschema show … | jq -c '.multi_tenant_spec' returns exactly:

{"tenant_id_column_name":"source_shard_id","tenant_id_column_type":"INT64"}

No new public vtctld GetVSchema surface is needed for this runbook gap.