fix(proxy): probe upstream liveness with TCP keepalive

[cleverhu]

What

Enable TCP keepalive probes on the sandbox proxy's connector: 3 s idle before the first probe, 1 s between probes, 2 retries.

Depends on #109. This branch is stacked on it because both touch build_proxy_client(); the diff shown here against main therefore also contains #109's commit. Please merge #109 first — this then rebases to a single-commit, three-line change.

Why

Tearing down a VM never delivers FIN or RST to the host side of a proxied connection. The socket stays open from the host's point of view, so a request addressed to a sandbox that is already gone hangs until some higher-level timeout fires instead of failing immediately.

Keepalive probes are what detect that silence. With the values above the failure surfaces in roughly 5 s, which is a prompt error for a caller and still far above any plausible pause in normal proxied traffic.

Related issue

None. Small, self-contained reliability fix on an existing code path.

Scope and non-goals

Included: keepalive parameters on the proxy connector.

Excluded:

• Retry or failover behavior when a probe does detect a dead peer — the request fails, as it should.
• Connection pooling. pool_max_idle_per_host(0) already handles the separate problem of interaction IPs being reused across runtime generations.
• Any other connector in the tree.

Design and behavior changes

Three constants next to the existing PROXY_CONNECT_TIMEOUT, applied via HttpConnector::set_keepalive{,_interval,_retries}.

Failure mode change: a request to a torn-down sandbox now returns a connection error after ~5 s instead of hanging. Callers that previously relied on a longer timeout to mask this will see the error sooner, which is the intent.

Compatibility and operations

• Public API or generated protocol: N/A.
• Configuration or defaults: no new config keys. The values are compile-time constants; if maintainers want them tunable I can move them into [orchestrator] or a proxy section.
• Snapshot manifest, artifact layout, or storage format: N/A.
• Upgrade and rollback: transparent both ways.
• Host requirements, permissions, ports, or dependencies: TCP_KEEPIDLE/TCP_KEEPINTVL/TCP_KEEPCNT are Linux socket options and AgentENV is Linux-only.

Validation

• make fmt
• make clippy
• make test-unit
• Relevant Rust integration tests
• make -C services test (required when services/ changes)
• Generated clients/server regenerated with the documented make target
• Documentation updated
• Benchmarks or performance comparison completed

Commands and results:

$ cargo fmt --all -- --check
(clean)

$ cargo clippy --workspace --all-targets -- -D warnings
Finished `dev` profile

$ cargo check -p agentenv --all-targets
Finished `dev` profile

Skipped checks and reasons:

• make test-unit not run. Observing this behavior in a test requires a peer that vanishes without sending FIN/RST, which is not reproducible with an in-process test server; it needs a real VM teardown or an injected netfilter drop.
• No benchmark: this is a failure-mode fix, not a throughput or latency change on the healthy path.

Risks and reviewer notes

The risk worth reviewing is the aggressiveness of the values. A 3 s idle threshold with 1 s probes means an idle-but-alive sandbox connection sends a probe every second. That is negligible traffic on a host-local link, but if the proxy is ever used across a slower network the numbers deserve revisiting.

The other question is whether ~5 s to detect is the right target. It is well under typical client timeouts while leaving room for a transient stall.

Most important file: src/api/proxy.rs.

Checklist

• The PR contains one coherent change and no unrelated formatting or refactoring.
• New behavior is covered by tests, or I explained why testing is impractical.
• Logs and examples contain no credentials, tokens, or private registry information.
• I did not manually edit generated code without updating its source and regenerating it.

[github-actions]

✅ OpenCodeReview: Review complete: 0 finding(s) across 3 selected item(s).