feat(tokenization): replace request_json with native proto fields in render RPCs

[sagearc]

RenderChatCompletionRequest and RenderCompletionRequest previously used a request_json string field, requiring callers to manually marshal OpenAI types to JSON. This replaces those fields with native proto types so callers work directly with structured proto messages.

RenderChatCompletionRequest now has messages, tools, chat_template, add_generation_prompt, continue_final_message, and chat_template_kwargs. RenderCompletionRequest now has prompt and model_name. ChatMessage is also extended to support multimodal content via content_parts (text + image_url blocks), consistent with the multimodal work in #219.

Ref: vllm-project/vllm#36102
Closes #449

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[Copilot]

Pull request overview

This PR updates the tokenizer gRPC API to replace request_json inputs for render RPCs with structured protobuf fields, and extends ChatMessage to represent multimodal content (text + image blocks) directly in proto.

Changes:

• Replace request_json in RenderChatCompletionRequest/RenderCompletionRequest with native proto fields (messages/tools/template options, prompt, etc.).
• Extend ChatMessage with content_parts plus new ContentPart/ImageUrl messages to support multimodal inputs.
• Regenerate protobuf Go bindings and update tokenizer UDS tests/code to use the new optional-string content accessor.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
pkg/tokenization/uds_tokenizer_test.go	Updates mock rendering to use GetContent() for optional proto field access.
pkg/tokenization/uds_tokenizer.go	Updates ChatMessage.Content assignment for new optional string field (currently introduces a pointer-to-range-variable bug).
api/tokenizerpb/tokenizer.proto	Adds multimodal message types; replaces JSON request fields with structured fields; adds render RPCs.
api/tokenizerpb/tokenizer.pb.go	Regenerated Go protobuf bindings reflecting the updated proto schema.
api/tokenizerpb/tokenizer_grpc.pb.go	Regenerated Go gRPC bindings; adds new RPC client/server methods.
api/indexerpb/indexer_grpc.pb.go	Regenerated header/version metadata only.
api/indexerpb/indexer.pb.go	Regenerated header/version metadata only.

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[vMaroon]

/lgtm
/approve