fix(retry): handle Retry-After zero delay

[Ram-blip]

This relates to...

Fixes the default retry handler so Retry-After: 0 is treated as a valid zero-delay retry instead of falling back to minTimeout.

Rationale

Retry-After delay-seconds can be zero. The current retry timeout selection only uses the parsed header value when it is greater than zero, so Retry-After: 0 falls through to the exponential backoff fallback.

This change adds an explicit zero-delay branch and changes invalid HTTP-date parsing to return null instead of 0, so invalid dates continue to use the fallback path.

Changes

• Preserve Retry-After: 0 as a zero-delay retry.
• Keep invalid Retry-After date values on the fallback retry path.
• Add a fake-timer regression test for the zero-delay case.

Features

N/A

Bug Fixes

Fixes default retry handling for Retry-After: 0.

Breaking Changes and Deprecations

N/A

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (cb4c2f1) to head (0cd0daf).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5519   +/-   ##
=======================================
  Coverage   93.44%   93.44%           
=======================================
  Files         110      110           
  Lines       37328    37330    +2     
=======================================
+ Hits        34881    34884    +3     
+ Misses       2447     2446    -1     

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