server : support jinja extra template kwargs (Qwen3 enable_thinking feature), from...

server : support jinja extra template kwargs (Qwen3 enable_thinking feature), from command line and from client (#13196)

* initial commit for handling extra template kwargs

* enable_thinking and assistant prefill cannot be enabled at the same time

* can set chat_template_kwargs in command line

* added doc

* fixed formatting

* add support for extra context in generic template init

* coding standard: common/chat.cpp

Co-authored-by: Georgi Gerganov <redacted>
* coding standard:  common/chat.cpp

Co-authored-by: Georgi Gerganov <redacted>
* Apply suggestions from code review

coding standard: cosmetic changes

Co-authored-by: Georgi Gerganov <redacted>
* fix merge conflict

* chat.cpp: simplify calls to apply to ensure systematic propagation of extra_context (+ the odd existing additional_context)

* normalize environment variable name

* simplify code

* prefill cannot be used with thinking models

* compatibility with the new reasoning-budget parameter

* fix prefill for non thinking models

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Co-authored-by: Georgi Gerganov <redacted>
Co-authored-by: Olivier Chafik <redacted>