Validate experimentation opt-out with shared assignment fixtures

[leoromanovsky]

Motivation

Customers need consistent experimentation exclusion across SDKs. Value-only assertions cannot distinguish an experimental control from an ordinary fallback with the same value.

Changes and Decisions

• Add the immutable experimentation opt-out contract from shared fixtures #48, pinned to 5e2430d9ad36644e2cecb6a32ddb8d05d7c98ac6.
• Expose actual Java evaluation details and explicit provider options through the existing test applications.
• Preserve large integer context values through the test application's OpenFeature conversion.
• Check assignment identity and metadata. Keep time-gated cases in fixed-clock SDK tests and guarded application tests.
• Keep manifest activation pending a released SDK version. Validate the draft with --runxfail; do not invent a release version.

Validation

Candidate implementation: dd-trace-java #12921, commit c4bfecd1555b533cbb95818c887b05886ad8c01d.

The local Java candidate passed all 11 force-enabled groups, covering 116 shared cases with no skips or expected failures. The candidate application run separately passed all 136 cases, including the 20 time-gated cases.

Existing dynamic-evaluation tests passed 24 cases. All 12 evaluation-event cases passed when force-enabled. The large-integer context regression failed on the old application image and passed after the harness correction.