feat:(vue): Optional injection key to allow multiple clients

[albertcito]

Summary

Previously, @urql/vue exposed a single client via Vue’s provide/inject under a fixed key, so apps could not cleanly use multiple urql clients (e.g., different GraphQL services) in the same app. This change adds optional custom injection keys so you can provide and consume several clients under different keys while keeping the default key '$urql' for existing usage.

Set of changes

• Package: @urql/vue (minor).
• packages/vue-urql/src/useClient.ts
  • Exported DEFAULT_KEY ('$urql').
  • provideClient(opts, key?), install(app, opts, key?), and useClient(key?) now accept an optional key (default DEFAULT_KEY). Provide and inject use this key so multiple clients can coexist.
• packages/vue-urql/src/useClientHandle.ts
  • useClientHandle(key?) now accepts an optional key (default DEFAULT_KEY), and uses useClient(key) internally so handles can target a specific client.
• Tests: packages/vue-urql/src/useClient.test.ts — added test “provides client with custom key to current component instance” that provides two clients (default and custom key) and asserts both are resolvable via useClient() and useClient(customKey).
• Docs: .changeset/some-bushes-smash.md describes the feature; docs/api/vue.md and docs/basics/vue.md may need follow-up to document the optional key and multi-client usage (not verified in this PR).

Resolves #3857

[changeset-bot]

🦋 Changeset detected

Latest commit: 0525c19

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@urql/vue	Minor

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