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]

The committers listed above are authorized under a signed CLA.

• ✅ login: ADITYA-CODE-SOURCE / name: Aditya Vishe (675f3c9)

[codecov]

Codecov Report

❌ Patch coverage is 95.08197% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.64%. Comparing base (b86f040) to head (675f3c9).
⚠️ Report is 8 commits behind head on main.

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%   91.64%   +0.03%     
- Complexity    10343    10365      +22     
============================================
  Files          1013     1013              
  Lines         27352    27424      +72     
  Branches       3215     3222       +7     
============================================
+ Hits          25057    25133      +76     
+ Misses         1567     1559       -8     
- Partials        728      732       +4     

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[opentelemetry-pr-dashboard]

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