[FFL-2929] Add composable assignment request transport

[leoromanovsky]

Motivation

Applications need opt-in assignment-request latency bounds and transient-failure retries without changing default request behavior. Advanced Android callers also need an idiomatic OkHttp transport seam for assignment-specific proxy, TLS, interceptor, and client policy.

Changes

• Add FlagsConfiguration.Builder.assignmentRequestTimeout(timeoutMs) and assignmentRequestRetryCount(retryCount), both defaulting to 0.
• Add assignment-only FlagsConfiguration.Builder.assignmentRequestCallFactory(Call.Factory).
• Keep SDK ownership of the URL, method, body, authentication, and custom headers; apply scalar timeout/retry policy to calls created by the supplied factory.
• Use a fresh one-shot Call per retry and cover the complete response body with an explicitly configured per-attempt timeout.
• Add construction-level builder wiring, real MockWebServer body-timeout tests, retry lifecycle tests, API fixtures, README, and changelog updates.

Decisions

• Upgrading preserves request behavior: omitted timeout and retry settings make one request, add no call timeout, and leave the supplied/platform transport authoritative.
• The custom Call.Factory affects assignments only. Exposure and evaluation uploads retain SDK transports, and the caller owns factory resources.
• Retry I/O failures, HTTP 408, and HTTP 5xx; never retry HTTP 429. Honor bounded HTTP 503 Retry-After plus full jitter.
• Preserve both legacy FlagsConfiguration.copy and copy$default JVM descriptors. The additive generated copy includes new fields, while the legacy overload preserves new transport settings.
• Expose OkHttp as an API dependency because Call.Factory is public.

Validation

• Exact CI-image Detekt and focused unit tests cover configuration, downloader retry policy, and production client transport wiring.
• Default-policy regression proves exactly one newCall/execute, no Call.timeout() access, and no retry delay.
• Isolated ktlint passes over every changed Kotlin file.
• Exact API-surface parser comparison, checked fixtures, Kotlin compilation, javap descriptor comparison, and a legacy binary consumer pass.
• Production downloader coverage includes delayed-body timeout, timeout 0 preserving custom client bounds, and successful delayed body with no SDK timeout.
• The generated transitive-dependency manifest includes the public OkHttp/Okio API dependencies.

Jira

FFL-2929

[codecov-commenter]

Codecov Report

❌ Patch coverage is 88.70968% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.41%. Comparing base (2dafcfa) to head (36e415d).

Files with missing lines	Patch %	Lines
...s/internal/net/PrecomputedAssignmentsDownloader.kt	91.49%	0 Missing and 4 partials ⚠️
...in/kotlin/com/datadog/android/flags/FlagsClient.kt	0.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3693      +/-   ##
===========================================
- Coverage    73.41%   73.41%   -0.00%     
===========================================
  Files          999      999              
  Lines        36564    36589      +25     
  Branches      6181     6190       +9     
===========================================
+ Hits         26842    26859      +17     
+ Misses        8000     7996       -4     
- Partials      1722     1734      +12     

Files with missing lines	Coverage Δ
...in/com/datadog/android/flags/FlagsConfiguration.kt	91.84% <100.00%> (+5.79%)	⬆️
...in/kotlin/com/datadog/android/flags/FlagsClient.kt	44.34% <0.00%> (-0.85%)	⬇️
...s/internal/net/PrecomputedAssignmentsDownloader.kt	92.16% <91.49%> (-4.14%)	⬇️

... and 43 files with indirect coverage changes

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[datadog-official]

✅ All CI checks and tests passed. Datadog automation helped this PR pass.

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

🔄 Datadog retried 1 test - 1 passed on retry

🎯 Code Coverage (details)
• Patch Coverage: 89.86%
• Overall Coverage: 71.22% (+0.12%)

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