chat : add new template for DeepSeek V4 Flash 0731 (#26398)

chat : add new template for DeepSeek V4 Flash 0731 (#26398)

* common/chat: update DeepSeek V4 templates

Align the DeepSeek V4 templates with the official encoders while keeping parser behavior out of this change.

- Default drop_thinking for DeepSeek V4 history so prior thinking is omitted unless preserve_reasoning is requested or tools are present.
- Add structured output response-format instructions to the V4 templates and pass the schema into template rendering.
- Add a separate Flash 0731 template for the updated high and max reasoning effort mapping.
- Cover reasoning effort, drop_thinking, structured output prompts, preserved reasoning, continuations, and empty tool arguments in template rendering tests.

Official references:
https://huggingface.co/deepseek-ai/DeepSeek-V4-Flash/blob/main/encoding/encoding_dsv4.py
https://huggingface.co/deepseek-ai/DeepSeek-V4-Flash-0731/blob/main/encoding/encoding_dsv4.py

Assisted-by: Codex
* Fix deepseek v4 0731 template selection

* remove unneeded lower normalization

* Fix DSML parser to consume the tool call separator

* address aldehir requests

* address aldehir comment