Backport: feat(provider/deepseek): support DeepSeek V4 reasoning effort

[github-actions]

This is an automated backport of #14743 to the release-v6.0 branch. FYI @ousugo
This backport has conflicts that need to be resolved manually.

git cherry-pick output

Auto-merging content/providers/01-ai-sdk-providers/30-deepseek.mdx
CONFLICT (content): Merge conflict in content/providers/01-ai-sdk-providers/30-deepseek.mdx
Auto-merging packages/deepseek/src/chat/deepseek-chat-language-model.test.ts
CONFLICT (content): Merge conflict in packages/deepseek/src/chat/deepseek-chat-language-model.test.ts
Auto-merging packages/deepseek/src/chat/deepseek-chat-language-model.ts
CONFLICT (content): Merge conflict in packages/deepseek/src/chat/deepseek-chat-language-model.ts
Auto-merging packages/deepseek/src/chat/deepseek-chat-options.ts
error: could not apply eda4373ca... feat(provider/deepseek): support DeepSeek V4 reasoning effort (#14743)
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config set advice.mergeConflict false"

[github-actions]

⚠️ Backport to release-v5.0 created but has conflicts: #15066

[github-actions]

🚀 Published in:

Package	Version
@ai-sdk/deepseek	2.0.33