Split integration test support into domain modules

[ryanfowler]

Summary

• Split the oversized tests/support/mod.rs into domain-specific modules for HTTP, TLS, HTTP/3, gRPC, DNS, WebSocket, proxy, PTY, terminal, update, auth, and common helpers
• Replace wildcard support imports in integration tests with explicit module imports
• Add a regression test that keeps the support entrypoint small and blocks wildcard support imports
• Update the repo guidance in AGENTS.md to reflect the new layout

Testing

• cargo fmt
• cargo clippy --locked --all-targets --all-features -- -D warnings
• cargo test --all-features --lib
• cargo test --all-features --test cli integration_support_stays_domain_split
• cargo test --all-features --test cli --test formatting --test grpc --test http --test network --test terminal --test update --test websocket -- --test-threads=1