Implement guardrail to check for existing branches

[shoumikhin]

Added a guardrail step to prevent overwriting existing branches in the Apple workflow to avoid accidental binaries overwrite on S3.

[pytorch-bot]

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❌ 2 New Failures, 1 Unrelated Failure

As of commit 3b7a11a with merge base 488d761 ():

NEW FAILURES - The following jobs have failed:

• trunk / test-arm-cortex-m-size-test (zephyr-preset) / linux-job (gh)
  RuntimeError: Command docker exec -t 9ce32b9726492fc3c27a311e1cbb4518fee9b1fb56f05bb921b15a39b7ba0035 /exec failed with exit code 1
• trunk / unittest-release / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_conv2d.py::TestConv2d::test_fp16_conv2d

UNSTABLE - The following job is marked as unstable, possibly due to flakiness on trunk:

• pull / android / run-emulator (gh) (#16137)
  Timeout waiting for emulator to boot.

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[Copilot]

Pull request overview

This PR adds a guardrail check to the Apple workflow that prevents overwriting existing branches and associated S3 binaries. The check runs early in the set-version job before expensive build operations begin, verifying that the generated swiftpm-{VERSION} branch doesn't already exist on the remote repository.

Key Changes:

• Added guardrail step in set-version job to check for existing remote branches
• Prevents workflow execution when a version-specific branch already exists
• Provides clear error message instructing users to delete the branch before re-running

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

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[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated 1 comment.

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[Copilot]

[nitpick] The step name "Guardrail" is vague. Consider a more descriptive name like "Check for existing branch" or "Prevent branch overwrite" to better indicate the step's purpose.

Suggested change

	- name: Guardrail
	- name: Check for existing branch

[GregoryComer]

Looks good to me. Copilot nit about the name seems reasonable but it's your call.