fix(ffe): make exposure event test deterministic

[sameerank]

Motivation

The FFE exposure-generation test intermittently timed out while waiting for an exposure. It used the generic test-flag key shared with other FFE coverage and did not confirm that the evaluation returned the configured variation. Although the original timeout was not conclusively traced to a flag-key collision, isolating the fixture removes that possible source of cross-test interference and makes evaluation failures distinguishable from exposure-delivery failures.

Changes

• Give this test a dedicated flag key to avoid collisions with other FFE tests.
• Verify the feature flag evaluated to the configured on variation before checking its exposure.
• Keep all failure-producing assertions and waits in the test body so manifest xfails remain effective.
• Add return type annotations to the modified setup and test methods.

Validation

• The focused exposure-generation test passed locally on the final commit with the configured weblog.
• The same test passed in the latest Python dev django-py3.13 CI job; that job failed on an unrelated FFE evaluation-metric test.
• Mypy passed (529 source files).
• Ruff format and lint passed.
• git diff --check passed.

./format.sh proceeded through the Python, YAML, and parser checks, then encountered an unrelated macOS Bash 3 set -u failure in utils/scripts/shellcheck.sh when expanding an empty $@.

[github-actions]

CODEOWNERS have been resolved as:

tests/ffe/test_exposures.py                                             @DataDog/feature-flagging-and-experimentation-sdk @DataDog/system-tests-core

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Keep exposure assertions out of setup

When this file is xfailed by a manifest for unsupported tracer versions (for example the file-level version gates in manifests/dotnet.yml, manifests/golang.yml, and manifests/python.yml), this assertion now runs inside the repository's custom setup loop in pytest_collection_finish before pytest executes the xfailed test. A timeout here is re-raised as a setup/collection failure instead of being reported as an expected test failure, so running those declared-unsupported versions can abort the scenario. Keep the wait/assertion in the test body, or make the setup path avoid failing for xfailed items.

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[datadog-prod-us1-6]

🎉 All green!

🧪 All tests passed
❄️ No new flaky tests detected

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