fix: score think_format_reward when <think> is prefilled in the prompt

[behroozazarkhalili]

think_format_reward rejected valid completions when the chat template had already placed the opening thinking tag in the prompt. I made the opening tag optional while keeping the closing tag required and rejecting nested opening tags. The reward test suite passes apart from skipped tests, and lint and format checks pass on both files.
Fixes #6966.

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Note

Medium Risk
Changes RL reward criteria for format shaping; broader acceptance (any completion with </think>) may shift GRPO training signal versus the old strict opening-tag rule.

Overview
think_format_reward no longer requires a leading <think> tag so GRPO runs that decode only generated tokens (with the opening tag prefilled in the chat template) can earn 1.0 on well-formed reasoning. The matcher still requires </think>, forbids nested <think>, and uses an optional (?:<think>)? prefix on the existing regex.

Docs and doctest examples describe this GRPO/prefill behavior. Tests add prefilled-prompt-style completions as valid, drop “closing tag without opening” from the invalid set, and document that a bare or text-mention </think> also scores 1.0.

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