Add pscale branch resize for Postgres branches

[nickvanw]

What

Adds pscale branch resize <database> <branch> --cluster-size <size> to resize a Postgres branch's cluster.

$ pscale branch resize my-db main --cluster-size PS_10_GCP_X86
Resize of branch main to PS-10 started (state: resizing).

It calls the new PostgresBranches.Resize client (planetscale-go v0.171.0), which hits the async branch change-request endpoint PATCH .../branches/{branch}/changes and returns a PostgresClusterResizeRequest. --format json prints the full change request.

MySQL databases are pointed at pscale keyspace resize with a clear error, since the branch-level cluster change endpoint is Postgres-only (the MySQL/Vitess resize stays at the keyspace level).

Notes

• --cluster-size takes the fully-qualified SKU name as shown by pscale size cluster list (e.g. PS_10_GCP_X86); the API rejects the short PS_10 form for this endpoint.
• A 204 No Content (config already matches) is surfaced as "already configured with cluster size …".

Testing

• go build ./... and go test ./... pass.
• Verified end-to-end against a live Postgres branch (mint-demo): PS-5 → PS-10 → PS-5 round trip completed, and a concurrent call correctly returned "cluster resize in progress".

Dependencies

• planetscale-go #318 (merged, released as v0.171.0). go.mod/go.sum bumped here.

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