Guard eager OpenFeature init and add detailed /ffe/evaluate for Go agentless configuration contract

[pavlokhrebto]

Motivation

FFL-2695 tracks the Go agentless configuration-source implementation in dd-trace-go. Until that lands, the parametric app's /ffe/start and /ffe/evaluate contract must match the one already implemented for Python (#7411), so tests/parametric/test_ffe/test_configuration_sources.py can run against a dev build of dd-trace-go and fail for the right reason.

Changes

• main.go: skip eager OpenFeature provider init when any DD_FEATURE_FLAGS_* config env var is set. Leave setup to /ffe/start.
• ffe.go: /ffe/start builds and sets the provider once, guarded by sync.Once. It returns 500 with an error body on failure. /ffe/evaluate uses the *ValueDetails APIs per variationType. It returns {value, reason, errorCode}. On error it falls back to {value: default, reason: "ERROR"}.

Workflow

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3. Mark it as ready for review
   • Test logic is modified? -> Get a review from RFC owner.
   • Framework is modified, or non obvious usage of it -> get a review from R&P team

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Reviewer checklist

• Anything but tests/ or manifests/ is modified ? I have the approval from R&P team
• A docker base image is modified?
  • the relevant build-XXX-image label is present
• A scenario is added, removed or renamed?
  • Get a review from R&P team

[github-actions]

CODEOWNERS have been resolved as:

utils/build/docker/golang/parametric/ffe.go                             @DataDog/system-tests-reviewers
utils/build/docker/golang/parametric/main.go                            @DataDog/system-tests-reviewers