Thinking model disabled assistant prefill (#15404)

* feat: Set enable_thinking IFF not disabled and supported

Branch: gabe-l-hart/thinking-model-disabled-agent-prefill

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* fix: Fix inverted logic condition for prefill error

Branch: gabe-l-hart/thinking-model-disabled-agent-prefill

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* fix: Always parse the enable_thinking kwarg to overwrite the default value

From what I can tell, this started as a Qwen3-specific keyword, but from
the use in `chat.cpp` translates this inputs.enable_thinking to the right
thinking kwarg for the given model, this is now more of a standardized
kwarg, so it should always override the default value when sent as part of
the chat_template_kwargs field in the API.

Branch: gabe-l-hart/thinking-model-disabled-agent-prefill

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* fix: Don't limit tempalte expansion check to jinja

With the use_jinja check, non-jinja models would enable thinking and always
fail assistant prefill

Branch: gabe-l-hart/thinking-model-disabled-agent-prefill

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* feat: Add the error text to json type errors in json_value

Branch: gabe-l-hart/thinking-model-disabled-agent-prefill

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* feat: Explicitly reject string values for "enable_thinking"

There are too many possible "truthy" / "falsy" strings and too many
ambiguous strings that don't have a clear truthy/falsy value, so the
simplest thing to do here is to reject the request. Ideally, this would be
a 422 (Unprocessable Entity), but right now it's coming back as a 500.

Branch: gabe-l-hart/thinking-model-disabled-agent-prefill

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* refactor: Move logic for detecting template enable_thinking support to common

Branch: gabe-l-hart/thinking-model-disabled-agent-prefill

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

* fix: Use raw pointer for common chat template function

Branch: gabe-l-hart/thinking-model-disabled-agent-prefill

Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>

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Signed-off-by: Gabe Goodhart <ghart@us.ibm.com>