Document client assignment request reliability

[leoromanovsky]

Motivation

Client applications need a visible way to protect assignment initialization from stalled requests without changing default SDK behavior. The primary setup path should show a bounded timeout, while the platform guides retain the complete timeout, retry, and lower-level transport building blocks.

This PR consolidates the cross-client documentation and supersedes #38869, #38870, #38871, #38872, and #38873.

Changes

• Add an early getting-started warning and a timeout-only browser setup example capped at 1,500 milliseconds.
• Add timeout-only setup examples for iOS, Android, Flutter, and Unity. Unity uses one second because its API accepts whole seconds.
• Document browser flagConfigurationFetch with withRetry(withTimeout(globalThis.fetch, timeoutMs), retries) in advanced configuration.
• Document opt-in scalar timeout/retry controls for iOS, Android, Flutter, and Unity in advanced configuration.
• Add composable assignment-only examples for iOS AssignmentRequestFetch, Android Call.Factory, Dart http.Client, and Unity IAssignmentRequestTransport.
• Clarify transport ownership, policy precedence, full-body timeout scope, fresh request attempts, and telemetry isolation.

Decisions

• Preserve default request behavior across every client: 0 disables the SDK timeout and retry policies, producing one initial request while underlying platform/custom transport limits remain authoritative.
• Show a timeout in primary setup guidance, keep retry configuration in advanced guidance, and keep every recommended timeout at or below 1,500 milliseconds.
• Retry network errors, timeouts, HTTP 408, and HTTP 5xx only when explicitly configured; never retry HTTP 429.
• Defer React Native documentation until the iOS and Android APIs are published and React Native can consume released native versions.
• Keep this PR draft until every documented native API is released.

Validation

• git diff --check passes.
• Vale over all client/getting-started pages reports 0 errors.
• Examples were checked against the exact public API fixtures and live draft PR heads.
• A full Hugo build is unavailable in this sparse documentation worktree because build dependencies and Makefile.config are absent.

Dependencies

1. Browser @datadog/openfeature-browser@1.4.0 is published to npm as latest.
2. iOS [FFL-2891] Add composable assignment request transport dd-sdk-ios#3167 and Android [FFL-2929] Add composable assignment request transport dd-sdk-android#3693 can land independently.
3. Flutter [FFL-2930] Add composable assignment request transport dd-sdk-flutter#1104 and Unity [FFL-2931] Add composable assignment request transport dd-sdk-unity#239 can land independently.
4. React Native follows only after released iOS and Android versions are available.
5. Merge this documentation PR after the documented SDK versions are released.

[github-actions]

Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/leo.romanovsky/document-flag-assignment-reliability/feature_flags/client/android
• https://docs-staging.datadoghq.com/leo.romanovsky/document-flag-assignment-reliability/feature_flags/client/flutter
• https://docs-staging.datadoghq.com/leo.romanovsky/document-flag-assignment-reliability/feature_flags/client/ios
• https://docs-staging.datadoghq.com/leo.romanovsky/document-flag-assignment-reliability/feature_flags/client/javascript
• https://docs-staging.datadoghq.com/leo.romanovsky/document-flag-assignment-reliability/feature_flags/client/unity
• https://docs-staging.datadoghq.com/leo.romanovsky/document-flag-assignment-reliability/getting_started/feature_flags/