docs(docs-infra): guard sandbox reset before initialization in playground

changeTemplate() was calling reset() on the sandbox before init() completed, causing a TypeError when spawning processes on an uninitialized WebContainer. Add isSandboxReady signal to skip reset until the sandbox is fully initialized.

PR Checklist

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• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/contributing-docs/commit-message-guidelines.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.dev application / infrastructure changes
• Other... Please describe:

What is the current behavior?

When the playground component initializes, an effect fires changeTemplate() which calls reset() on the NodeRuntimeSandbox before init() has completed. This causes a TypeError: Cannot read properties of undefined (reading 'spawn') because the WebContainer instance hasn't been created yet. This error is reproducible on angular.dev/playground in production.

Issue Number: N/A

What is the new behavior?

A isSandboxReady signal is added that is set to true only after init() completes. changeTemplate() now checks this signal before calling reset(), preventing the TypeError during initial load.

Does this PR introduce a breaking change?

• Yes
• No

Other information