fix: respect NODE_TLS_REJECT_UNAUTHORIZED environment variable

[0x5457]

fix: #2601

After testing, I found that the fetch behavior is as follows:

• Fetch only uses the NODE_TLS_REJECT_UNAUTHORIZED environment variable when the rejectUnauthorized property is completely absent from agent options
• If rejectUnauthorized is explicitly set to any value (including undefined), the environment variable is ignored

Also should I can create test certificates in the test-data directory to avoid introducing the selfsigned library

[k8s-ci-robot]

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