fix(security): bound inbound request framing

[seonghobae]

No description provided.

[seonghobae]

Current exact head is 3830f38. Review evidence: 1483 full tests on #765 substrate, 38 framing/security focused tests after final change, compileall, ruff, actionlint, osv-scanner no issues, and diff check passed. This PR intentionally remains stacked on #765 until parent reaches protected main; then retarget/rebase and rerun all hosted gates before normal merge.

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[seonghobae]

@opencode-agent Review exact current HEAD 3830f38 only. Verify strict fixed-length framing, duplicate/malformed/oversized Content-Length rejection, unsupported Transfer-Encoding rejection, bounded socket read timeout restoration, close-on-error behavior, and RFC 9112 evidence. Submit a formal exact-head verdict without modifying the branch.

[seonghobae]

Exact-head review at 0ec115134d21d1fbee17cc5431e1c9433667ee5a reproduced a real failure:

• pytest -q tests/test_inbound_request_total_deadline.py tests/test_inbound_request_framing.py -> 1 failed, 18 passed.
• The new slow-progress regression fails because _read_json only sets a per-read socket timeout; it never checks a total monotonic deadline, so byte trickling can extend the request indefinitely.

I am preserving this branch and preparing a stacked root-cause fix that recalculates the remaining deadline before every read and fails closed with 408. Do not merge 0ec1151 until the exact-head test is green.

[seonghobae]