feat(zenmux): add custom/deepseek/deepseek-v4-flash

[xiaojiezj]

Add custom/deepseek/deepseek-v4-flash to ZenMux provider.

• base_model = "deepseek/deepseek-v4-flash"
• Reasoning support with toggle + effort options
• Cost: input $0.14, output $0.28, cache_read $0.0028 per million tokens

Data sourced from https://zenmux.ai/api/anthropic/v1/models

[github-actions]

Action items

• [high] [possible mistake] providers/zenmux/models/custom/deepseek/deepseek-v4-flash.toml:2 - Check: reasoning_options must match the provider’s Anthropic request surface, not an OpenAI-compatible sibling. Why: This entry sets npm = "@ai-sdk/anthropic" and api = "https://zenmux.ai/api/anthropic/v1", but copies toggle + effort low|medium|high from the OpenAI-compatible deepseek/deepseek-v4-flash entry. ZenMux’s own Anthropic notes document thinking.type enabled|disabled with required budget_tokens (≥1024), not effort; native DeepSeek Anthropic documents output_config.effort = high|max (budget ignored). The proposed effort values match neither surface. Action: Re-audit against ZenMux Anthropic reasoning docs / a real Anthropic Messages request for this model ID, then set options to the verified controls (likely toggle + budget_tokens, or native DeepSeek’s toggle + effort high|max if ZenMux passthrough is confirmed). Do not copy the OpenAI-compatible sibling.
• [medium] [possible mistake] providers/zenmux/models/custom/deepseek/deepseek-v4-flash.toml:4 - Check: interleaved must describe this inference surface’s response shape. Why: field = "reasoning_content" is an OpenAI-style field and matches the OpenAI-compatible ZenMux DeepSeek entry, but no ZenMux Anthropic-surface models (@ai-sdk/anthropic + /api/anthropic/v1) declare interleaved; Anthropic Messages exposes thinking blocks, not reasoning_content. Action: Confirm the Anthropic endpoint actually returns interleaved reasoning_content; if not, remove [interleaved].

[rekram1-node]

/oc fix this pr, interleaved is incorrect here, if it is using the anthropic api then reasoning_content isnt valid.