Migrate to new tekken crate

[jorge-menjivar]

Purpose

Migrate the Rust workspace's Mistral Tekken tokenizer dependency from tekken-rs 0.1.1 to tekken 0.2.0.

I am the author of tekken-rs and recently renamed it to simply tekken to make it easier to import, find, and align with its library name. Since crates.io has no rename mechanism, this meant publishing a new crate; tekken-rs 0.1.2 is the final release under the old name and will not receive further updates.

Version 0.2.0 of tekken also adds:

• Support for v15 of the tokenizer
• Image tokenization support
• Audio and image dependencies gated behind feature flags

Notes on this PR:

• No code changes are needed: tekken-rs's library name was already tekken, so all existing use tekken::... imports keep working. Since the crate is now actually named tekken, the package = "tekken-rs" alias in the workspace dependency goes away too.
• The existing default-features = false was a no-op on 0.1.1 (it had no feature flags, the audio dependencies were unconditional), but on 0.2.0 it takes effect: since the Rust workspace only uses tekken for text tokenization, hound, rubato, rustfft, and ndarray drop out of the dependency tree, and the new image feature is never pulled in.

Test Plan

cd rust
cargo test -p vllm-tokenizer
cargo test --workspace

Test Result

- cargo test -p vllm-tokenizer: 38 passed, 0 failed
- cargo test --workspace: 841 passed, 0 failed (all 12 crates build and pass)

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