Add configurable sleep for slow CA order processing

[denisfoulon]

This PR keeps the change minimal and shell-compatible.

It adds configurable waiting for slow ACME order processing:

• LE_PROCESSING_MIN_SLEEP
• LE_MIN_RETRY_SLEEP
• LE_MAX_RETRY_AFTER

This helps with CAs returning Retry-After: 0 or taking longer to finalize certificates with many SANs.

Tested successfully with a slow CA on a certificate containing many SANs.

[denisfoulon]

Related to #5233, #6103, #6763.

This PR improves handling of slow order processing and retry timing for ACME servers returning Retry-After: 0 or taking longer to finalize certificates with many SANs.

This follow-up PR removes the unrelated shebang change from the previous attempt and targets dev only. The diff is now limited to the retry/sleep handling for slow CA order processing.

[Copilot]

Pull request overview

This PR aims to make ACME order polling more resilient to slow CA processing and problematic Retry-After behavior by introducing configurable minimum sleeps and retry thresholds.

Changes:

• Introduces LE_MIN_RETRY_SLEEP to enforce a minimum delay when handling Retry-After (notably for 503 overload responses).
• Introduces LE_MAX_RETRY_AFTER (currently only reflected in log messaging) intended to cap how long the client will wait before giving up.
• Adds LE_PROCESSING_MIN_SLEEP during order “processing” polling to add additional delay between retries.

[denisfoulon]

Hi everyone,

I've updated the PR to address the points raised during the review:

Robustness: Added ${_retryafter:-0} to prevent arithmetic syntax errors if the header is missing.

Consistency: Replaced hardcoded 600 thresholds with the ${LE_MAX_RETRY_AFTER} variable.

Best Practices: Switched from sleep to the internal _sleep function.

Coverage: Updated the logic to ensure the configured minimum sleep is respected even if the CA returns Retry-After: 0.

[denisfoulon]

I've applied the formatting fixes (arithmetic spacing and indentation) to comply with shfmt. Tests should be greener now! Thanks for your help on this.