[Security] Harden client ID generation in ExtensionServerClient

[gonzaloriestra]

This pull request hardens the identifier generation inside ExtensionServerClient in ui-extensions-server-kit. It uses a cryptographically secure pseudo-random number generator (CSPRNG) if available, with a fallback to Math.random otherwise.

How to test your changes?

Run the unit tests:
pnpm --filter @shopify/ui-extensions-server-kit vitest run

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PR created automatically by Jules for task 16599871158628811553 started by @gonzaloriestra

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