fix(ai): expose low thinking level for DeepSeek V4 Flash on opencode/opencode-go (#8181)

DEEPSEEK_V4_FLASH_THINKING_LEVEL_MAP (which enables the low effort level)
was only applied to deepseek/deepseek-v4-flash, so the same model served
through opencode and opencode-go only offered off/high/max. models.dev
reports reasoning_options effort values of low/high/max for these
providers, and the live opencode-go API accepts reasoning_effort=low
(verified against https://opencode.ai/zen/go/v1).

Apply the flash map to every deepseek-v4-flash variant on the deepseek,
opencode, and opencode-go providers. qwen-token-plan is left unchanged
since its gateway support for the low effort level is not verified.