Reject malformed Connect END_STREAM JSON

[fallintoplace]

Summary

• reject malformed Connect END_STREAM JSON instead of defaulting to a clean success
• use one strict END_STREAM parser for both server-streaming and client-streaming responses
• add regression coverage for malformed END_STREAM frames after a valid message

Why

Malformed END_STREAM payloads were being parsed with unwrap_or_default(), which turned invalid JSON into an empty terminal response and let corrupted or truncated stream terminators look successful.

Impact

• malformed Connect END_STREAM frames now surface as terminal internal errors
• client-streaming keeps response headers attached on the new error path

Validation

• cargo test -p connectrpc malformed_end_stream
• cargo test -p connectrpc end_stream

[github-actions]

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