[NV] Speedbench-al: fix --chat-template-kwargs default quoting so thinking-on cells run

[qiching]

The collector set CHAT_TEMPLATE_KWARGS_ON="${VAR:-{...}}", but bash ends the parameter expansion at the first }, so the trailing } is appended as a literal. It worked locally because the var was unset (default path lands on a single brace), in CI the workflow passes the var in, so the value became {...}} invalid JSON. Fixed by moving the default into its own variable so there are no brace-literals in the expansion.

Parent PR: #1650

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Note

Low Risk
Single-line bash env default change in a benchmark collector script; no production serving or auth paths affected.

Overview
Fixes thinking-on SPEED-Bench AL collection in CI by correcting how the default CHAT_TEMPLATE_KWARGS_ON JSON is assigned in dsv4_fp4_b300_vllm.sh.

The previous inline default inside ${CHAT_TEMPLATE_KWARGS_ON:-{...}} was parsed incorrectly by bash (expansion stops at the first }), which could yield malformed JSON when the workflow exports the variable. The default is now stored in DEFAULT_CHAT_TEMPLATE_KWARGS_ON and referenced via ${CHAT_TEMPLATE_KWARGS_ON:-$DEFAULT_CHAT_TEMPLATE_KWARGS_ON}, so --chat-template-kwargs gets valid {"thinking": true, "reasoning_effort": "high"} for thinking-on cells.

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