test(ffe): validate Go agentless EVP egress

[leoromanovsky]

Motivation

FFL-1491

Go needs system-test coverage proving exposure and flagevaluation delivery through Agent, direct, and serverless-sidecar EVP routes.

Changes

• Add the shared flagevaluation egress contract for Agent, direct, and serverless-sidecar scenarios.
• Enable Go agentless exposure and flagevaluation route coverage in the manifest.
• Assert direct intake host, response status, API-key authentication, and absence from the unused route.
• Keep buffered writers alive through flush and isolate aggregate counts by targeting key.
• Trust the checked-in MITM CA in the Go test image while retaining TLS verification.

Decisions

• Validate exposure and flagevaluation together in each deployment topology.
• Accept the API key as either its expected value or the capture layer's redacted value.
• Preserve exact payload, aggregate-count, deduplication, and route-exclusion assertions.
• Keep this draft stacked on pavlo.khrebto/FFL-2695/go-agentless-configuration.

Validation

• ./format.sh
• ./build.sh golang -w net-http using the local Go EVP branch artifact
• FEATURE_FLAGGING_AND_EXPERIMENTATION: 2 passed
• FEATURE_FLAGGING_AND_EXPERIMENTATION_AGENTLESS_DIRECT: 2 passed
• FEATURE_FLAGGING_AND_EXPERIMENTATION_AGENTLESS_SERVERLESS: 2 passed

[linear-code]

FFL-1491

[github-actions]

CODEOWNERS have been resolved as:

manifests/golang.yml                                                    @DataDog/dd-trace-go-guild
tests/ffe/test_exposure_egress.py                                       @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
tests/ffe/test_flag_eval_evp.py                                         @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
tests/test_the_test/scenarios.json                                      @DataDog/system-tests-core
utils/build/docker/golang/net-http.Dockerfile                           @DataDog/dd-trace-go-guild @DataDog/system-tests-core

[datadog-official]

🎉 All green!

❄️ No new flaky tests detected

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