test(ffe): cover billed configuration source opt-ins

[leoromanovsky]

Motivation

CDN polling and Agent Remote Configuration delivery both have customer billing impact. The shared configuration-source contract must prove that neither path starts implicitly, and that the provider kill switch does not silently cancel a source the customer explicitly selected.

Node implementation: DataDog/dd-trace-js#9413

Changes

• uses DD_FEATURE_FLAGGING_PROVIDER_ENABLED in the source-selection suite
• asserts explicit remote_config advertises the FFE capability and applies FFE_FLAGS
• asserts explicit agentless reaches the controlled CDN mock without advertising the FFE RC capability
• adds provider-disabled cases proving explicit CDN polling and RC subscription still run
• adds an unset-source case proving no CDN request and no FFE RC capability
• adds exact Content-Length to mock responses and self-tests it so Node 18 fetch can consume the controlled JSON:API payload

Decisions

• An unset source is the billing-safe default and starts neither delivery path.
• Provider registration and delivery-source activation are independent assertions.
• The Node manifest is not activated here because the first released tracer version containing feat(openfeature): make billed configuration sources opt-in dd-trace-js#9413 is not fixed yet.
• The mock continues to validate the managed dd_env query and now also emits a complete HTTP response framing contract.

Validation Evidence

• 7/7 Node configuration-source selection cases pass against the local tracer.
• 3/3 mock backend self-tests pass.

[github-actions]

CODEOWNERS have been resolved as:

tests/parametric/test_ffe/test_configuration_sources.py                 @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
tests/test_the_test/test_mock_ffe_agentless_backend.py                  @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core
utils/docker_fixtures/_mock_ffe_agentless_backend.py                    @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core