Fix app operator snapshot truncation

[yvette-carlisle]

Summary

• switch accepted operator HTTP sockets back to blocking mode before serving responses
• keep request-header read timeout while avoiding truncated large app snapshot writes
• add a regression test for large /api/operator-snapshot responses from nonblocking listeners

Validation

• cargo +nightly fmt --all -- --check
• cargo vstyle curate --language rust --workspace --all-features
• cargo test -p decodex operator_state_endpoint
• cargo check -p decodex
• runtime smoke: full decodex serve --listen-address 127.0.0.1:8912 returned complete 356346-byte /api/operator-snapshot JSON with 7 projects and no truncation