feat: Support usePost for FDv2 polling and streaming requests

[kinyoklion]

Summary

The FDv2 data system carried an unwired usePost flag on each mode-definition entry: the requestor and streaming factories honored it, but the built-in modes hardcoded it to false and no configuration exposed it. Under FDv2 the SDK always sent the evaluation context in the URL with GET, and useReport was silently ignored.

• DataSystemConfig.usePost (default false) switches every FDv2 polling and streaming source to POST with the context JSON in the request body, on the /sdk/poll/eval and /sdk/stream/eval paths. It replaces the per-entry usePost fields on PollingInitializer, StreamingInitializer, PollingSynchronizer and StreamingSynchronizer, which were not reachable through configuration (the data system is in early access, with no compatibility guarantees). The setting travels through SourceFactoryContext, so built-in modes and overrides behave the same.
• On web the browser EventSource cannot send a POST. When usePost is set on such a platform the data system drops the streaming sources from every mode and logs a warning, so a streaming mode polls instead. The platform capability is a new streamingPostSupported default next to the existing streamingReportSupported.
• useReport together with a DataSystemConfig now logs a warning that the option is ignored and points at usePost.
• The contract test service declares client-use-post and maps clientSide.usePost onto DataSystemConfig.usePost.
• The FDv1 fallback source also keeps the context out of the URL under usePost. The FDv1 endpoints do not accept POST (production returns 404 for POST /msdk/evalx/context and 200 for REPORT), so the fallback poller uses REPORT with the context JSON body when usePost is set, and GET otherwise.

The POST request variants come from the v3 harness change in launchdarkly/sdk-test-harness#445. With a harness built from that branch the full v3 suite passes against this branch: 844 total, 823 ran, 20 of them POST subtests. CI runs the v2 harness, which this change does not affect.

Also verified against production with a mobile key through the contract test service: in polling mode behind a logging proxy the SDK sent POST /sdk/poll/eval with the context JSON body and received the environment's flags; in a streaming-only mode (no polling initializer, no cache) with usePost the stream alone delivered the same 183 flags, matching the GET control runs.

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Note

Overview
Adds DataSystemConfig.usePost (default off) so FDv2 polling and streaming send the evaluation context in a POST body instead of a GET URL path. The old per-mode-entry usePost fields are removed; the setting flows through SourceFactoryContext to all FDv2 sources.

On web, where EventSource cannot POST, enabling usePost strips streaming sources from resolved modes, logs a warning, and leaves polling so streaming modes degrade to poll. streamingPostSupported is added to platform defaults (true on IO, false on JS).

FDv1 fallback honors the same flag via REPORT + JSON body (FDv1 does not accept POST). useReport with an FDv2 data system now logs that it is ignored and points to usePost.

The Flutter contract test service advertises client-use-post and maps harness clientSide.usePost onto DataSystemConfig.usePost.

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[joker23]

nit: not understanding this comment change.

[kinyoklion]

There are built-in defaults that get filtered based on what the platform supports.