[BUG] Keep the curl retry deadline stable within an attempt

[shixi-li]

Fixes #4403

NextRetryTime() drew a new jitter sample on every call, so polling the same retryable request moved its deadline and biased the realized delay toward the lower end of the backoff band.

Changes

• calculate and store one retry deadline when each curl attempt completes
• make the stored deadline visible before the synchronous Response event and reuse it for scheduling
• preserve valid Retry-After deadlines and use one stable jittered backoff when the header is missing or invalid
• cover repeated reads, response-event visibility, invalid-header fallback, and later attempts

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed (no public API surface is changed)

Tests

• focused curl retry deadline and exponential-backoff tests
• full curl_http_test suite
• ./ci/do_ci.sh cmake.maintainer.sync.test
• ./ci/do_ci.sh cmake.test
• ./ci/do_ci.sh cmake.exporter.otprotocol.test
• ./ci/do_ci.sh format
• markdownlint .

Coverage notes

The focused cases exercise the shared completion path through synchronous Send(). The existing full client suites cover async operation, but this change does not add an end-to-end timing assertion for the pending retry queue. Existing HTTP-date and over-max-backoff Retry-After behavior is unchanged. Local runtime validation was on Linux/amd64.

[codecov]

Codecov Report

❌ Patch coverage is 86.36364% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.85%. Comparing base (3603e13) to head (b699ff2).

Files with missing lines	Patch %	Lines
ext/src/http/client/curl/http_operation_curl.cc	86.37%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4452      +/-   ##
==========================================
+ Coverage   82.63%   82.85%   +0.23%     
==========================================
  Files         512      512              
  Lines       20138    20145       +7     
==========================================
+ Hits        16639    16690      +51     
+ Misses       3499     3455      -44     

Files with missing lines	Coverage Δ
...lemetry/ext/http/client/curl/http_operation_curl.h	91.31% <ø> (ø)
ext/src/http/client/curl/http_operation_curl.cc	62.99% <86.37%> (+2.40%)	⬆️

... and 1 file with indirect coverage changes

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