[codex] fix DeepSeek image attachments

[brandonkachen]

What changed

• Normalize DeepSeek chat request messages before sending them to the direct DeepSeek API.
• Convert OpenRouter-style multimodal content arrays into text-only content for DeepSeek, preserving text and adding an explicit notice when image/file parts are omitted.
• Widen the server chat message type to represent text/image/file content parts.
• Add integration coverage for DeepSeek image attachment request serialization.

Why

Freebuff DeepSeek requests with image attachments were forwarding image_url content parts to the direct DeepSeek backend. The current DeepSeek chat completion API accepts text content for user messages, so those image parts caused provider-format failures.

Validation

• CI=true bun test web/src/llm-api/__tests__/deepseek-image-compat.integration.test.ts
• CI=true bun run --cwd=web typecheck
• bunx prettier --check web/src/llm-api/deepseek.ts web/src/llm-api/types.ts web/src/llm-api/__tests__/deepseek-image-compat.integration.test.ts