fix: move Stripe webhook helpers to separate file to fix Next.js rout… #5729

Job	Run time
build-and-check	2m 39s
test-integration-packages/billing	1m 2s
test-integration-packages/internal	1m 12s
test-integration-sdk	36s
test-integration-web	42s
test-integration-packages/agent-runtime	48s
test-packages/agent-runtime	42s
test-.agents	40s
test-web	46s
test-common	40s
test-packages/billing	38s
test-cli	55s
test-sdk	47s
test-packages/internal	51s
test-integration-cli	40s
test-integration-common	40s
	14m 18s

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