feat(server): RETRY-spec conformance in FDv1 streaming and polling (SDK-2789)

[tanderson-ld]

Summary

Implements RETRY-spec conformance in the Java server SDK's FDv1 streaming and polling data sources. Per epic SDK-2775 and analogous to Go's reference implementation SDK-2788.

Behavioral change: HTTP responses that today cause a data source to permanently stop (notably 401, 403, other 4xx) and TLS/certificate validation failures are no longer terminal. Streaming enters an extended backoff regime (5 min → 1 hr, doubling); polling continues at its configured cadence with extended-regime waits between failing polls. Recovery from either regime uses RETRY §1.8's healthy-operation reset (60 s of continuous connectivity).

• Ticket: SDK-2789
• Guide: `server-sdk-guide.md` in sdk-scratchpad.
• Implementation plan: `SDK-2789/plan.md`.
• Go reference retro: `SDK-2788/retro.md`.

Scope: FDv1 streaming and polling data sources under `lib/sdk/server/`. FDv2 is deliberately out of scope for this epic (SDK-2775 defers FDv2 to a future epic); nothing in `datasourcev2/` or the DataSystem-related code paths is touched.

Highlights

• `FailureClass` enum + local classifier. HTTP status per §1.6, transport exception per §1.7 (TLS/cert → UNEXPECTED). Classifier stays local to java-server-sdk; no changes to `launchdarkly-java-sdk-internal`.
• `PollingStrategy` state-machine encapsulation. State: `n`, `initialDelay`, `maxDelay`, `priorPollWasSuccessful`. Methods: `onFailure(class)`, `onSuccess()`, `nextWait()`. Wait floor `max(pollInterval, T − J)` per RETRY §1.4.4 override. Two-consecutive-successes reset per §1.8.1.
• `PollingProcessor` rewire. From `scheduleAtFixedRate` to a self-driven loop using `strategy.nextWait()`. Removed the `State.OFF` permanent-stop path.
• `StreamProcessor` regime switching. Consumes okhttp-eventsource's new `setInitialRetryDelayMillis` / `setMaxRetryDelayMillis` (see launchdarkly/okhttp-eventsource#109). On UNEXPECTED classification, transitions retry timing into the extended regime; on `activeSince` elapsed ≥ `retryResetInterval`, transitions back to normal. Library's built-in healthy-op reset is disabled so the SDK owns regime state.
• Test-only builder knobs. Package-private setters on the concrete Impl classes; extends the existing `pollIntervalWithNoMinimum` precedent.
• Cross-module bridge. New `com.launchdarkly.sdk.server.internal.DataSourceInternalHelpers` (delegating through package-private `ComponentsInternalBridge`) gives cross-module callers access to the test-only knobs. Package name signals internal-only surface.
• Contract test service. Declares `retry-conformance-fdv1-streaming` and `retry-conformance-fdv1-polling` capabilities. New RETRY-related fields on `SdkConfigStreamParams` and `SdkConfigPollingParams`. `SdkClientEntity` wires them via the internal bridge.

Testing

• Unit tests: full suite BUILD SUCCESSFUL. New tests: `FailureClassTest` (22 cases), `PollingStrategyTest` (9 cases). 401/403 tests renamed and rewritten to assert extended-regime retry instead of permanent stop.
• Contract tests via `launchdarkly/sdk-test-harness#404` (v2 branch, RETRY-conformance tests): 4889 tests pass end-to-end, including the extended-regime paired-bound assertion.

Test plan for reviewers

• Confirm the FailureClass mapping (§1.6 / §1.7) matches your expectations, particularly TLS/cert = UNEXPECTED.
• Confirm the `.internal` package as the marker for cross-module test-only access is acceptable as a new convention.
• Sanity-check `StreamProcessor.handleError` — classifier + regime-transition + healthy-op-reset ordering.
• The library's built-in healthy-op reset is now disabled in `StreamProcessor`, since the SDK owns regime state. Confirm this is the right call.
• Sanity-check the mixed-classification transition test in `PollingStrategyTest.mixedClassificationNormalThenUnexpectedStartsAtExtendedInitial`.

Dependency

CI will be red on this PR until launchdarkly/okhttp-eventsource#109 is released to Maven Central. The `setInitialRetryDelayMillis` / `setMaxRetryDelayMillis` methods this SDK relies on are only in that PR's branch. Once released, bump `okhttp-eventsource` version in `lib/sdk/server/build.gradle`.