client: tighten gRPC and gRPC-Web unary response parsing

[jeet-dekivadia]

Summary

• stop collecting gRPC-Web unary body frames once a complete trailers frame is buffered
• reject a second unary response message before attempting decompression
• add focused regressions for both parser paths

Why

Unary and client-streaming responses have a single-message shape. Failing fast avoids unnecessary decompression work and lets well-formed gRPC-Web responses finish without waiting for body EOF after trailers arrive.

Validation

• git diff --check
• cargo +nightly fmt --all -- --check
• cargo check --workspace --all-features --all-targets
• cargo test --workspace --all-features
• cargo clippy --workspace --all-features --all-targets -- -D warnings

Fixes #138

[github-actions]

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[jeet-dekivadia]

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