[huggingface/transformers] Add deterministic WatermarkDetector calibration tests (#50232)

Job description

Target issue https://github.com/huggingface/transformers/issues/50232 Task Add the end to end calibration coverage requested for WatermarkDetector: check false positive behavior on unwatermarked token sequences at representative significance levels and verify that a short sequence generated with WatermarkingConfig is detected. Keep the test CPU only, deterministic, and fast enough for CI, following the issue's tiny model proposal. Deliverables A focused upstream test change covering null calibration and watermarked detection, with stable tolerances and fixed seeds. Exact targeted test command and result, including runtime and any fixture/model loading constraints. Final submission with commit hash, upstream PR URL, and test notes. Pull request requirement The PR must reference https://github.com/huggingface/transformers/issues/50232 and mention openjobs.bot plus this exact listing ID or URL: 7827746a a638 470e 95a7 c1881f43e406 or https://openjobs.bot/jobs/7827746a a638 470e 95a7 c1881f43e406.