test: add reason strings to bare #[ignore] attributes

[saitejagirada]

Summary

Fill every bare #[ignore] and #[ignore] // comment attribute under src/ and tests/ with a reason string in the #[ignore = "..."] form, matching the convention used by the other ~350 ignore attributes in the tree. In every case the reason text was already adjacent in the file — a module doc comment, a MODEL_DIR const, an eprintln! skip message, or a trailing comment.

Closes #1698

Changes

Eight files, 23 sites:

File	Sites	Reason
tests/deepseek_v4_real_model.rs	5	real-model heavy (~151 GB checkpoint)
tests/qwen38_mtp_chain_parity.rs	6	real-model heavy; needs Qwen 3.5-family checkpoint
tests/got_ocr_prompt_parity.rs	3	needs a GOT-OCR 2.0 checkpoint on disk
tests/qwen3_omni_moe_parity.rs	1	diagnostic; needs real Qwen3-Omni checkpoint
tests/ernie4_5_moe_vl_parity.rs	1	diagnostic; needs real ERNIE-4.5 MoE VL checkpoint
src/models/diffusion_gemma/tests.rs	2	real-model regression test; needs DiffusionGemma checkpoint
src/downloader/tests.rs	1	hits the real Hugging Face Hub; CI must not depend on network
src/tokenizer/tiktoken_tests.rs	4	requires model files on disk (moved from trailing // comment)

Verification

# Zero bare #[ignore] remaining:
grep -rn '^\s*#\[ignore\]\s*$' src/ tests/ --include='*.rs'
# (empty)

# Zero trailing-comment style remaining:
grep -rn '#\[ignore\] //' src/ tests/ --include='*.rs'
# (empty)

No logic changes — purely mechanical text moves into attribute reason strings.

Acceptance Criteria (from #1698)

• Zero bare #[ignore] remains under src/ and tests/