chore(frontend): bump vllm rust crates to cc706b05

[RicardoMin]

Summary

This PR updates the Rust vLLM dependencies required by the frontend and keeps the change separate from #865.

• Bump the five directly pinned vLLM Rust crates from 8e61b646 to cc706b05.
• Refresh Cargo.lock with the dependency closure required by the new revision.
• Add EngineCoreOutput::ec_transfer_params: None for compatibility with the updated API.

Dependency Update

Crate	Previous revision	New revision
vllm-chat	8e61b646	cc706b05
vllm-engine-core-client	8e61b646	cc706b05
vllm-server	8e61b646	cc706b05
vllm-text	8e61b646	cc706b05
vllm-tokenizer	8e61b646	cc706b05

Validation

Scope	Command	Result
vLLM tokenizer tests	cargo test --release --locked --features test-utils -p vllm-tokenizer --lib	57 passed, 0 failed
vLLM text tests	cargo test --release --locked -p vllm-text --lib	70 passed, 0 failed, 1 ignored
PegaInfer frontend tests	cargo test --release --locked -p pegainfer-frontend --lib	63 passed, 0 failed
PegaInfer frontend compilation	cargo check --release --locked -p pegainfer-frontend --all-targets	Passed
Patch formatting	git diff --check upstream/main...HEAD	Passed

The tokenizer test suite includes the zero-generated-token flush regressions:

Regression test	Result
flush_without_push_token_does_not_leak_prompt	Passed
flush_without_push_token_does_not_leak_undecodable_prompt_tail	Passed

The single ignored vllm-text test requires network access to Hugging Face and is marked as too slow for CI.