Support image media in DeepSeek chat requests

[Lubaoshuai]

Fixes #6978

Problem

DeepSeekChatModel.createRequest only read message.getText() for USER messages, so any Media attached to a UserMessage was silently dropped: a vision-capable model such as deepseek-flash received a text-only request, with no error telling the caller the image was discarded.

Solution

Map user message media to the OpenAI-compatible multimodal content array that the DeepSeek vision documentation describes (text and image_url blocks), mirroring the approach the Mistral AI adapter already uses:

• DeepSeekApi.ChatCompletionMessage.content is now a polymorphic String-or-ContentChunk-list with a custom serializer/deserializer, so text-only messages serialize exactly as before while multimodal user messages serialize as [{"type":"text",...},{"type":"image_url",...}].
• Added ContentChunk, TextChunk and ImageUrlChunk (whose ImageUrl supports base64 data URIs for byte[] media; String media data is passed through as a URL).
• DeepSeekChatModel maps user message media on both the synchronous and streaming paths (both go through createRequest).
• Response handling is unchanged: assistant content still always deserializes as a String.

Tests

New tests in DeepSeekChatCompletionRequestTests:

• URL media produces a content array with a text chunk and an image_url chunk
• byte[] media is inlined as a data:image/png;base64,... URI
• text-only user messages keep plain string content (backward compatibility)
• wire-format test asserting the serialized request message JSON

Full spring-ai-deepseek module run: 48 tests, all passing.

[lilingshan]

In addition, the default model should be changed to DEEPSEEK_FLASH, as DEEPSEEK_V4_FLASH has now been officially replaced by the official release.

[Lubaoshuai]

Thanks @lilingshan — done. DeepSeekApi.DEFAULT_CHAT_MODEL now resolves to ChatModel.DEEPSEEK_FLASH (deepseek-flash), and the two integration tests that referenced the previous default were updated accordingly. DEEPSEEK_V4_FLASH remains available as an explicit option for users who still need it.

Also noticed the DCO check was stuck pending because the three review-follow-up commits were missing Signed-off-by — the branch has been rewritten with the sign-off added (force-pushed, no other changes), and the checks are re-running.