Respect Retry-After in OTLP HTTP senders

[ADITYA-CODE-SOURCE]

Summary:

• honor OTLP/HTTP Retry-After headers in the OkHttp and JDK HTTP senders
• support both delay-seconds and RFC1123 HTTP-date values, with existing exponential backoff as the fallback
• add shared parser coverage plus sender and OTLP integration tests for seconds/date/malformed cases

Fixes #8449

Testing:

• ./gradlew :exporters:common:test --tests "*RetryUtilTest"
• ./gradlew :exporters:sender:okhttp:test --tests "*RetryInterceptorTest"
• ./gradlew :exporters:sender:jdk:test --tests "*JdkHttpSenderTest"
• ./gradlew :exporters:otlp:all:testOkHttpVersionLATEST --tests "OtlpHttp"
• ./gradlew :exporters:otlp:all:testJdkHttpSender --tests "OtlpHttp"
• ./gradlew :exporters:common:spotlessCheck :exporters:sender:okhttp:spotlessCheck :exporters:sender:jdk:spotlessCheck :exporters:otlp:testing-internal:spotlessCheck :exporters:otlp:all:spotlessCheck :exporters:otlp:all:checkstyleTestOkHttpVersionLATEST :exporters:otlp:all:checkstyleTestJdkHttpSender

[linux-foundation-easycla]

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[codecov]

Codecov Report

❌ Patch coverage is 94.82759% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.61%. Comparing base (b86f040) to head (1249450).

Files with missing lines	Patch %	Lines
.../io/opentelemetry/exporter/internal/RetryUtil.java	86.66%	1 Missing and 1 partial ⚠️
...orter/sender/okhttp/internal/RetryInterceptor.java	93.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #8633   +/-   ##
=========================================
  Coverage     91.60%   91.61%           
- Complexity    10343    10360   +17     
=========================================
  Files          1013     1013           
  Lines         27352    27396   +44     
  Branches       3215     3219    +4     
=========================================
+ Hits          25057    25098   +41     
- Misses         1567     1568    +1     
- Partials        728      730    +2     

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