Fix extra_buffer_lazy guard bypass

[ispobock]

Motivation

#30437 blocks extra_buffer_lazy for the DFLASH/DSPARK spec algorithms (their verify bypasses prepare_mamba_track_for_verify, so the lazy pending slot never receives the checkpoint scatter yet is still promoted in result processing). The guard was nested under if view.speculative_num_draft_tokens is not None, but _validate_mamba_extra_buffer runs before handle_speculative_decoding resolves that value, and DFLASH derives it from --speculative-dflash-block-size. So --speculative-algorithm DFLASH --speculative-dflash-block-size N --mamba-radix-cache-strategy extra_buffer_lazy (without an explicit --speculative-num-draft-tokens) sees None at validation and the guard is skipped; a lowercase dflash also slips past the uppercase string compare.

This keys the check on the algorithm name (present and case-normalized at validation time) instead of speculative_num_draft_tokens, so the unsupported combination is rejected regardless of how the block size is passed or cased.

Verified the previously-bypassing configs (DFLASH+block_size, lowercase dflash, DSPARK) now raise at startup, while EAGLE / NEXTN + extra_buffer_lazy stay allowed.

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CI States

Latest PR Test (Base): ❌ Run #29830661282
Latest PR Test (Extra): ❌ Run #29830661097

[gemini-code-assist]

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