Add retry mechanism for scan enqueue failures

[wterpstra]

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Description

This PR adds a configurable retry mechanism for scan creation when the queue capacity is reached (error code 142). The implementation includes exponential backoff to handle temporary queue capacity issues gracefully.

Changes include:

• New --scan-enqueue-retries flag to control number of retry attempts (default: 0, no retries)
• New --scan-enqueue-retry-delay flag to set base delay between retries (default: 5 seconds)
• Exponential backoff: delay doubles with each retry attempt (e.g., 5s, 10s, 20s)
• Queue capacity error detection and retry logic in scans-http.go:40
• Mock test support for queue capacity errors
• Verbose logging for retry attempts

Type of Change

• New feature (non-breaking change which adds functionality)

Related Issues

Fixes #1349

Checklist

• I have performed a self-review of my code
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)
• Any dependent changes have been merged and published in downstream modules
• I have updated the CLI help for new/changed functionality in this PR (if applicable)
• All active GitHub checks for tests, formatting, and security are passing
• The correct base branch is being used

Additional Notes

The default behavior remains unchanged (0 retries), ensuring backwards compatibility. Users can opt-in to retry functionality by setting the flags or environment variables (CX_SCAN_ENQUEUE_RETRIES, CX_SCAN_ENQUEUE_RETRY_DELAY).