Make test hook setup safe across repeated runs

[metascroy]

Summary:
Repeated GTest invocations and the XCTest bridge retain the process-global hook registry, so unconditional test-provider registration fails on subsequent suite setup. Register once per process while retaining the assertion that the initial registration succeeded. Add a regression for repeated setup without changing the production one-provider contract or duplicate-provider rejection coverage.

Leave the disabled Xcode-staging configuration unchanged.

Differential Revision: D123082075

[pytorch-bot]

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❌ 3 New Failures

As of commit 6e64ee2 with merge base 305638c ():

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• Cadence Build & Test / Resolve CI docker image / resolve (gh)
  ##[error]Refusing to check out fork pull request code from a 'pull_request_target' workflow. This workflow runs with the base repository's GITHUB_TOKEN, secrets, default-branch cache scope, and runner access. Fetching and executing a fork's code in that trusted context commonly leads to "pwn request" vulnerabilities. To opt in, review the risks at https://gh.io/securely-using-pull_request_target and set 'allow-unsafe-pr-checkout: true' on the actions/checkout step.
• Cadence Build & Test / vision-build / Resolve CI docker image / resolve (gh)
  ##[error]Refusing to check out fork pull request code from a 'pull_request_target' workflow. This workflow runs with the base repository's GITHUB_TOKEN, secrets, default-branch cache scope, and runner access. Fetching and executing a fork's code in that trusted context commonly leads to "pwn request" vulnerabilities. To opt in, review the risks at https://gh.io/securely-using-pull_request_target and set 'allow-unsafe-pr-checkout: true' on the actions/checkout step.

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[meta-codesync]

@metascroy has exported this pull request. If you are a Meta employee, you can view the originating Diff in D123082075.

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