test(ffe): frame mocked CDN responses for fetch clients

[leoromanovsky]

Motivation

Node.js built-in fetch rejects close-delimited HTTP response bodies when the connection terminates without an explicit framing signal. The agentless UFC mock merged in #7315 returned JSON bodies without Content-Length, so otherwise valid Node system tests failed while decoding the response.

Changes

• Add Content-Length to mocked UFC configuration responses.
• Add Content-Length to the mock's JSON status and error responses.
• Verify the advertised configuration length in the fixture self-test.

Decisions

• Keep this follow-up limited to standards-compliant HTTP response framing; the JSON:API payload, dd_env query contract, authentication checks, and request tracking remain unchanged.
• Validate framing in the fixture's own TEST_THE_TEST scenario so SDK system tests can rely on the controlled CDN surface.

Validation

uv run --python 3.12 pytest -q -S TEST_THE_TEST tests/test_the_test/test_mock_ffe_agentless_backend.py
3 passed in 2.34s

Required by DataDog/dd-trace-js#9397.

[github-actions]

CODEOWNERS have been resolved as:

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