[Spec Decoding] Add DFlash e2e tests and Buildkite CI

[aaronzhfeng]

Description

Add e2e tests and Buildkite CI for DFlash block-diffusion speculative decoding. The DFlash model/proposer were added in #1868, and pipeline integration in #1869. This PR adds the test coverage and CI.

Verified on both TPU v4 and v5p across 9 datasets (math, code, chat) with Qwen3-4B target + z-lab/Qwen3-4B-DFlash-b16 draft, achieving 3x average speedup.

Files:

• tests/e2e/test_speculative_decoding.py -- add test_dflash_correctness (Qwen3-4B + DFlash draft, output correctness) and test_dflash_performance (1.5x speedup threshold)
• .buildkite/features/Speculative_Decoding-_DFlash.yml -- Buildkite CI pipeline for DFlash correctness and performance, modeled after Eagle3's Speculative_Decoding-_Eagle3.yml

Tests

pytest tests/e2e/test_speculative_decoding.py::test_dflash_correctness
pytest tests/e2e/test_speculative_decoding.py::test_dflash_performance

Checklist

• I have performed a self-review of my code.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have made or will make corresponding changes to any relevant documentation.