Preserve Kafka setting during ClickPipe updates

[sdairs]

Summary

• preserve the current kafka_read_committed value when updating unrelated ClickPipe settings
• fail instead of fabricating a value when the tolerant settings response omits the required field
• add subprocess and wiremock coverage for the settings GET to PUT request flow

Verification

• cargo test -p clickhousectl
• cargo clippy -p clickhousectl --all-targets -- -D warnings
• cargo fmt --all --check

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.