client: error on truncated Connect streaming responses

[Yong-yuan-X]

Summary

• return an error when a Connect streaming response body ends before an END_STREAM envelope is received
• add a regression test for a response that yields a data envelope and then EOFs without END_STREAM
• document the client-visible behavior change in the changelog

Closes #140.

Testing

• cargo check -p connectrpc --all-features --all-targets
• cargo clippy -p connectrpc --all-features --all-targets -- -D warnings
• cargo test -p connectrpc connect_server_stream_truncated_after_data_errors
• cargo check -p connectrpc --no-default-features
• cargo test -p connectrpc --no-default-features
• RUSTDOCFLAGS='-Dwarnings' cargo doc -p connectrpc --all-features --no-deps

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