feat: add cluster mode support and harden registration validation

[janisto]

Adds managed cluster support (clientMode: 'cluster'), strengthens runtime validation (invalid clientMode, empty namespace, and mixed default/namespaced registration rejection), updates typings/tests accordingly, and refreshes README with upgrade notes and corrected async usage/examples.

Fix #31

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[Copilot]

Pull request overview

This PR extends @fastify/valkey-glide to support managed cluster clients via a new clientMode option, tightens registration/option validation (including rejecting mixed default + namespaced registrations), and updates TypeScript typings, tests, and documentation accordingly.

Changes:

• Add clientMode: 'standalone' | 'cluster' to create either GlideClient or GlideClusterClient, with runtime validation.
• Enforce stricter registration rules (no mixed default + namespaced usage in the same Fastify instance tree) and validate namespace (empty string rejected).
• Update TS types + tsd assertions, README examples/upgrade notes, and add/adjust unit tests.

Reviewed changes

Copilot reviewed 5 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
index.js	Adds cluster client creation and mixed-mode detection using an internal namespace-container marker; adds option validation.
types/index.d.ts	Updates public TS types to represent fastify.valkey as a union (root client vs namespaced container) and adds cluster option typings.
types/index.test-d.ts	Updates tsd tests for the new typings and adds negative assertions that require narrowing.
test/index.test.js	Adds unit tests for empty namespace, mixed-mode rejection, and cluster client creation; fixes one close-client test access.
README.md	Adds upgrade notes and new examples (cluster + async/await usage).
package.json	Bumps package version and broadens the allowed @valkey/valkey-glide semver range.

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