feat: Add client-use-post capability and POST request variants

[kinyoklion]

Summary

FDv2 client-side SDKs can send the evaluation context in the request body with POST (the client-side FDv2 usePost option) instead of encoding it into the GET path. The harness could only exercise GET and, for SDKs that declare client-use-report, REPORT.

• Adds the client-use-post capability and a clientSide.usePost configuration parameter.
• The polling and streaming request tests (method and headers, URL path, query parameters, context properties, etag) gain a POST variant next to GET and REPORT when the SDK declares the capability. POST requests must use the FDv2 /sdk/poll/eval and /sdk/stream/eval paths and carry the context JSON in the body. Existing test names are unchanged; the new variants are additive.
• The auto-env-attribute request-body tests use REPORT when the SDK declares client-use-report, otherwise POST when it declares client-use-post, and skip when it declares neither.
• Documents the capability and parameter in docs/service_spec.md, including the promise that the harness never sets useReport and usePost in the same configuration, so test services do not need a precedence rule. A unit test pins that promise on the request-method configurer.

Verified against the Flutter client SDK contract test service with usePost support (launchdarkly/flutter-client-sdk, branch rlamb/sdk-3051/fdv2-use-post): the full v3 suite passes, 844 total, 823 ran, including 20 POST subtests.

---

Note

Overview
Adds client-use-post and clientSide.usePost so FDv2 client SDKs can be contract-tested when they send streaming/polling flag requests as POST with context in the body (parallel to existing REPORT / useReport).

When a test service declares the capability, shared poll/stream suites gain additive POST subtests: FDv2 /sdk/poll/eval and /sdk/stream/eval, body assertions via sendsContextInBody() (covers both REPORT and POST). Auto-env-attribute body tests pick REPORT if client-use-report is declared, else POST if client-use-post, otherwise skip.

docs/service_spec.md documents the capability and the rule that the harness never sets useReport and usePost together; a unit test locks that on withFlagRequestMethod.

^{Reviewed by Cursor Bugbot for commit 6b70b95. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

^{❌ Bugbot Autofix is OFF. To automatically fix reported issues with cloud agents, have a team admin enable autofix in the Cursor dashboard.}

^{Reviewed by Cursor Bugbot for commit 6b70b95. Configure here.}

[cursor]

Roku path matcher ignores POST

Medium Severity

The Roku polling URL-path matcher still treats only REPORT as a body method, so a POST variant falls through to the FDv1 GET prefix. availableFlagRequestMethods still adds POST when the SDK declares client-use-post, so those assertions check the wrong route instead of the FDv2 /sdk/poll/eval path.

 

^{Reviewed by Cursor Bugbot for commit 6b70b95. Configure here.}