[Rust Frontend] Use oss-harmony for Harmony output processing

[BugenZhao]

Purpose

Switch the library used for Harmony output processing from Inferact/openai-harmony (introduced in #46696) to oss-harmony, a more actively maintained fork of the original library.

This gives the Rust frontend two practical benefits:

• picks up upstream Harmony bug fixes and improvements
• embeds the tokenizer vocabularies in the library itself, removing the runtime download path and the related reqwest / TLS backend dependency from Harmony output processing

This PR is the dependency-only base for the follow-up GPT-OSS Harmony renderer work.

Test Plan

• cargo nextest run --manifest-path rust/Cargo.toml -p vllm-chat output::harmony
• cargo check --manifest-path rust/Cargo.toml -p vllm-chat --locked

Test Result

Both commands passed locally.

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[BugenZhao]

This change is part of the following stack:

• [Rust Frontend] Use oss-harmony for Harmony output processing #46799 ◀
  • [Rust Frontend] Add Harmony Renderer for GPT-OSS #46800

_{Change managed by git-spice.}