Add chat template kwargs to C API

[jennyf19]

Preserve the existing API while allowing typed JSON context values for model-specific chat templates.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

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[Copilot]

Pull request overview

This PR extends the tokenizer C API to accept additional, typed JSON context values (“template kwargs”) when applying chat templates, while preserving the legacy OrtxApplyChatTemplate entry point.

Changes:

• Adds a new C API function OrtxApplyChatTemplateWithOptions that accepts template_kwargs as a JSON object string.
• Updates chat template rendering to merge template_kwargs into the Minja context while preventing overrides of core context keys (messages, tools, add_generation_prompt).
• Adds new unit tests covering typed kwargs, core-context override prevention, and invalid kwargs handling.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
test/pp_api_test/test_tokenizer_chat.cc	Adds tests for typed template kwargs behavior, override-prevention, and invalid JSON errors.
shared/api/tokenizer_impl.h	Extends TokenizerImpl::ApplyChatTemplate signature to accept template_kwargs.
shared/api/chat_template.cc	Implements parsing/merging of template_kwargs into the template context with core-key precedence rules.
shared/api/c_api_tokenizer.cc	Introduces OrtxApplyChatTemplateWithOptions and routes legacy API through it.
include/ortx_tokenizer.h	Exposes the new API in the public header with documentation.

Suppressed comments (1)

shared/api/c_api_tokenizer.cc:486

• OrtxApplyChatTemplateWithOptions can dereference a null tokenizer when tokenizer == nullptr but template_str != nullptr. The current guard only rejects the case where both tokenizer and template_str are null, but token_ptr is always derived from tokenizer, so this can crash instead of returning kOrtxErrorInvalidArgument.

  if (tokenizer == nullptr && template_str == nullptr) {
    ReturnableStatus::last_error_message_ = "both tokenizer and template_str are null, no template to apply";
    return kOrtxErrorInvalidArgument;
  }

  if (input == nullptr || output == nullptr) {
    ReturnableStatus::last_error_message_ = "Invalid argument";
    return kOrtxErrorInvalidArgument;
  }

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