Dual-path retry: exponential backoff + CDN-driven httpConfig support

[MichaelGHSeg]

Summary

Rewrites HTTPClient.Upload() with a structured dual-path retry system and adds support for reading retry configuration from CDN settings (integrations["Segment.io"].httpConfig).

Retry logic (HTTPClient.cs)

• 429 + Retry-After header: sleep for the specified duration, does NOT consume the retry budget. Bounded by MaxRateLimitRetries and MaxRateLimitDuration (default 12h). Retry-After: 0 is treated as a 1-second floor to prevent busy-loops.
• Other retryable errors (5xx, 408, 410, 460): counted exponential backoff (base 500ms, 2×, cap 60s). Bounded by MaxRetries and MaxTotalBackoffDuration (default 12h).
• Non-retryable errors: discard immediately.
• Returns true on all terminal failure paths so EventPipeline removes the batch file instead of re-uploading indefinitely.
• New configurable properties: MaxRetries, MaxRateLimitRetries, BackoffEnabled, RateLimitEnabled, BaseBackoffMs, MaxBackoffMs, MaxRetryAfterCapSeconds, StatusCodeOverrides.
• Adds X-Retry-Count header on retry attempts.

CDN-driven config (SegmentDestination.cs)

• Reads httpConfig.backoffConfig and httpConfig.rateLimitConfig from CDN settings and applies them to the HTTP client after settings arrive.
• statusCodeOverrides uses dynamic key enumeration (matching the analytics-kotlin approach) instead of a hardcoded status code list — any valid HTTP status code (100–599) in the CDN config is respected.
• Uses CultureInfo.InvariantCulture for double.TryParse to handle non-English locales correctly.
• Maps maxRetryInterval to MaxRetryAfterCapSeconds (per-request sleep cap, not total budget).

E2E cli (e2e-cli/)

• CapturingLogger filters to final-failure messages only ("Retries exhausted", "Max total backoff") to avoid false failures from transient per-attempt errors.
• Uses enrichment callbacks for userId injection instead of pre-calling Identify.
• Adds 2s wait after new Analytics() when AUTO_SETTINGS=true so CDN settings arrive before the first upload.
• Enables retry and retry-settings test suites.

Test plan

• dotnet test passes
• E2E basic,retry,retry-settings suites pass (68/68)

[MichaelGHSeg]

Closing this as superseded, with the surviving piece carried forward in #147.

Context. This branch and #144 were never stacked — both branched independently off 85f025e ("Add e2e-cli for E2E testing (#142)", May 11). None of this PR's commits (bb0e925 May 15 → b50d4de May 21) appear in #144's history (6732ff6 May 28 → 11198cc Jun 4). They were parallel implementations of the same goal, and #144 landed first.

What #144 already covers. #144 reimplemented this ground more thoroughly (1,929 insertions vs 326 here): the full Retry/ package — RetryStateMachine, RetryConfig, RetryState, RetryStateStorage, HttpConfigParser, TimeProvider — plus EventPipeline/SyncEventPipeline.UpdateHttpConfig, e2e-cli/Program.cs, and tests. e2e-cli/e2e-config.json is byte-identical between the two branches. The Unity DoPost signature change here is no longer needed either: #144 added a virtual 3-arg DoPost that delegates to the abstract 2-arg one, so the sample still compiles.

What was still missing, and where it went. #144 left no user-facing entry point — RateLimitConfig, BackoffConfig, RetryConfig and HttpConfig were all internal, so retry behaviour could only come from CDN settings. This PR fixed that with individual MaxRetries / MaxTotalBackoffDuration / MaxRateLimitDuration knobs on Configuration. That shape matches analytics-python, but not Kotlin (Configuration.httpConfig: HttpConfig?) or Swift (public func httpConfig(_:)) — the SDKs #144 was written to align with.

#147 closes the gap in the Kotlin/Swift shape instead: it makes HttpConfig and friends public and adds Configuration.HttpConfig. #148 then adds generic Retry-After/529 handling on top.

Nothing here is lost — the branch remains if we want to revisit the per-knob API separately.