Add customer keyspace rollout concurrency settings

[darwich6]

Customers cannot currently configure the existing keyspace shard rollout concurrency from the CLI. Add pscale keyspace update-settings --max-rollout 1..32 and --reset-max-rollout, with readback through keyspace settings.

The vendored API client preserves the difference between an omitted setting, an explicit integer, and JSON null for reset. Human output shows the configured integer, defaulting to 1 when unset; JSON returns the stored nullable value. Settings PATCHes now include only selected groups, avoiding unrelated durability updates on Metal while preserving unspecified flags within VReplication updates. The new flags reject interactive-mode combinations.

Related: https://github.com/planetscale/issues/issues/1469. Requires https://github.com/planetscale/api-bb/pull/22013 to be deployed before CLI release.

Validation:

• Full go test ./..., go build -trimpath ./..., and go vet ./... passed.
• Formatting and git diff --check passed.
• Local CLI → running development API smoke test passed for set/read/reset and preserving unrelated settings; original local value restored. Direct HTTP validation also rejected 33 with HTTP 422.
• GitHub CI passed: Build, Test, & Lint; lint (including golangci-lint); and dependency license verification.
• Development Singularity is mocked; actual shard rollout concurrency remains untested on a real Vitess development cluster.