server : fix server_tokens::push_back infinite loop and placeholder copy

[sanjeevafk]

Overview

Fixes #29865 in server_tokens::push_back(const server_tokens & tokens):

• Insert tokens in bulk using this->tokens.insert(...) rather than calling the single-token overload push_back(tokens[i]), which throws an error on LLAMA_TOKEN_NULL placeholders.
• Add the missing ++it in the media map loop to fix an infinite loop when copying media chunks.
• Make the argument a const reference and use it->second.get().

Note: Dropped the keep_first change from this PR to avoid overlap with #24076.

Additional information

Tested locally appending server tokens with media chunks; confirmed the loop terminates properly and placeholders copy without throwing.

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: YES - Used an AI assistant to help investigate the hang and trace token copy logic. Manually tested and reviewed.

[ggml-gh-bot]

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[sanjeevafk]

Updated the PR description and commit message to match the template and project guidelines.

[ServeurpersoCom]

Hi, the keep_first fix is already covered by #24076, open since June

[sanjeevafk]

@ServeurpersoCom Thanks for catching that! I didn't realize #24076 had already addressed the keep_first boundary.

I've reverted the keep_first change here to avoid any overlap, and updated this PR to focus strictly on fixing the push_back infinite loop and LLAMA_TOKEN_NULL placeholder copy for #29865.

[ServeurpersoCom]

Note that on master this path is only reached by format_prompt_rerank with text-only tokens, so this is a latent fix with no reachable repro today.

[sanjeevafk]

Ok then, I will leave it up to the maintainers whether to merge it proactively or wait till the multimodal rerank expands. Thanks for the catch.