Add EAGLE-3 end-to-end speculative-decode test

[digantdesai]

Tiny matching target + draft on CPU drive the shifted method-flow the runner
implements -- prefill, draft chain, target_verify, accept, reseed -- and the
generated tokens are checked against greedy decoding (lossless by construction).
Forced-acceptance cases pin the partial, full, and accepted-EOS paths plus the
one-token budget; the random-weight loop alone can leave them uncovered.

This covers the decoding algorithm on CPU only; the CUDA/AOTI export (export.py)
and the C++ runner are exercised manually. Registered in pytest.ini next to the
draft/speculator unit tests.

Authored with assistance from Claude Code.

[digantdesai]

Stack from ghstack (oldest at bottom):

• -> Add EAGLE-3 end-to-end speculative-decode test #20157
• Add the EAGLE-3 speculative-decoding runner (CUDA) #20156
• Add the EAGLE-3 speculator CUDA export #20155
• Add a weight-stationary INT4 GEMM for small M #20154
• Add the Eagle3Speculator module for speculative decoding #20153
• Add KV cache to the EAGLE-3 draft head #20152
• Add Gemma4 EAGLE-3 eager reference #20151
• Add Gemma4 EAGLE-3 hidden-state taps #20150
• Add EAGLE-3 draft head #20149

[pytorch-bot]

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