[Bugfix][Rust Frontend] Backport integer KV handoff conversion to v0.29

[alec-flowers]

Purpose

Backport the integral protobuf-number conversion from #54814 to releases/v0.29.0. Main already contains the fix and handoff coverage; the release branch diverged before that commit. No overlapping release backport was found.

v0.29.0's Rust gRPC receiver converts pp_size: 1 into JSON 1.0. NIXL then fails at range(remote_pp_size) with TypeError: 'float' object cannot be interpreted as an integer, preventing native prefill/decode transfer.

This takes only the numeric conversion from #54814, preserving safe integral values recursively while leaving fractional expiry timestamps and out-of-range values as floats. One request-conversion regression checks the KV metadata delivered to the engine.

Test plan and results

• The new regression fails after removing the normalization guard; with the backport, all 14 grpc::convert::tests pass:
  cargo +1.95 test --locked --manifest-path rust/Cargo.toml -p vllm-server --lib grpc::convert::tests
• rustfmt +1.95 --edition 2024 --check rust/src/server/src/grpc/convert.rs and git diff --check pass.
• Real native gRPC P/D validation: Qwen/Qwen3-0.6B, vLLM 0.29.0, Dynamo 1.5.0.dev20260910, NIXL 1.3.2, separate prefill/decode engines sharing an RTX 6000 Ada. The same 641-token prompt with eight requested tokens and KV load failure policy fail fails on the stock binary with the TypeError above and zero completion tokens. Replacing only the Rust binary with this backport passes with eight completion tokens and finish reason length.
• No model, kernel, or sampling behavior is changed; validation covers the serving protocol regression rather than model quality.

AI assistance was used to investigate, implement, and test this draft.