Rename RESTATE_API_KEY env var to RESTATE_AUTH_TOKEN

[pcholakov]

Use RESTATE_AUTH_TOKEN everywhere for consistency, based on customer feedback.

Summary

• Renames the RESTATE_API_KEY environment variable to RESTATE_AUTH_TOKEN across the five CDK Lambda examples (TypeScript, Go, Java, Kotlin, Python), the Next.js template, and the integration test runner.
• The Restate CLI and the docs/UI consistently use RESTATE_AUTH_TOKEN for the API key already; only these examples were the outlier. A customer flagged the inconsistency between docs pages and tooling, which surfaced when configuring deployments end-to-end.

Files touched

• typescript/integrations/deployment-lambda-cdk/{lib/lambda-ts-cdk-stack.ts,README.md}
• go/integrations/go-lambda-cdk/{cdk/go-lambda-cdk-stack.ts,README.md}
• java/integrations/java-gradle-lambda-cdk/{cdk/lambda-jvm-cdk-stack.ts,README.md}
• kotlin/integrations/kotlin-gradle-lambda-cdk/{cdk/lambda-jvm-cdk-stack.ts,README.md}
• python/integrations/deployment-lambda-cdk/{lib/lambda-python-stack.ts,README.md}
• typescript/templates/nextjs/app/actions.ts
• .tools/run_node_tests.sh

This is a straight rename with no backward compatibility shim: callers must update their environment to use the new variable name.

Test plan

• CI builds for all five CDK examples pass with RESTATE_AUTH_TOKEN=key_test (the test runner was updated to set the new name)
• Manual cdk synth of the TypeScript stack with RESTATE_AUTH_TOKEN set produces a valid template
• Stack now fails with the expected error message when neither RESTATE_ENV_ID nor RESTATE_AUTH_TOKEN are set
• Companion docs PR in restatedev/docs-restate is merged so the CDK code block on docs/cloud/connecting-services stays in sync

[pcholakov]

CI checks are failing for unrelated reason, addressed by #390