fix(host-agent): reject oversized JSON bodies

[tang-vu]

Summary

• reject JSON request bodies larger than the host-agent MAX_BODY limit with HTTP 413
• close the HTTP connection before any oversized payload bytes are consumed
• read the declared length directly once it has passed the bound

Why this matters

Every host-agent JSON mutation endpoint calls read_json_body() or read_optional_json_body(). Both readers previously consumed min(Content-Length, MAX_BODY) and attempted to parse that prefix. A caller declaring an oversized body could therefore get inconsistent results (including acceptance when the available prefix happened to be valid JSON), while unread bytes remained on a persistent HTTP connection and could corrupt framing for the next request.

Root cause: MAX_BODY was used as a truncation length instead of an admission limit. The invariant is that a JSON request is either read exactly as declared within the configured bound, or rejected before consumption with the connection closed.

This is runtime correctness at the authenticated host-agent boundary; it is not presented as an unauthenticated remote security issue.

Overlap check

Searched open and closed PR titles for host-agent body, Content-Length host agent, MAX_BODY JSON, and request body too large. No semantic match exists. I also inspected every open PR touching ods/bin/ods-host-agent.py; current work there concerns OpenCode lifecycle, GPU/runtime planning, rootless reachability, PID writes, and remote-provider state transactions, not HTTP body framing.

Merged file history was also checked; the nearest body-size work is the endpoint-specific .env update cap, which does not protect the shared JSON readers.

Regression test

The parameterized boundary test calls both shared readers with Content-Length = MAX_BODY + 1 and a syntactically valid prefix. It asserts 413, Connection: close state, and that zero payload bytes were consumed.

Pre-fix evidence: both cases returned {}. Post-fix validation:

• pytest tests/test_host_agent.py -q — 259 passed, 4 skipped
• python -m py_compile ../../../bin/ods-host-agent.py tests/test_host_agent.py — passed
• git diff --check — passed

Tradeoffs and rollback

Oversized requests now require a new connection after the 413 response; this is intentional because the body is not drained. Valid requests at or below MAX_BODY are unchanged. Revert commit 0fdb3dd1 to roll back; no state or schema migration is involved.

Generated with Codex

Batch compatibility

Validated as an independent ten-PR batch from upstream main 6ff9b4fc5190099705043acaab7e9b6ad9c8b8f1. The final PR heads merged without conflicts in this order: #2964 -> #2965 -> #2967 -> #2969 -> #2970 -> #2971 -> #2972 -> #2973 -> #2974 -> #2975. The resulting local synthetic merge head is 4ae60eadad9696a9accb735aad71af87d2be802d.

Combined validation on that exact tree:

• Dashboard API boundary suites: 323 passed, 4 skipped.
• Token Spy suite: 36 passed, 1 skipped.
• Privacy Shield suite: 55 passed.
• APE suite: 47 passed.
• Python compile checks and git diff --check: passed.

The scopes are behaviorally independent. The stated order is the tested rollback/merge sequence for shared-file changes; each PR remains individually useful and revertible.