fix: bound reference HTTP ingress

[imran-siddique]

What changed

• validate Content-Length as bounded decimal input before reading
• return structured 400 responses for malformed UTF-8/JSON instead of escaping the handler
• apply a ten-second socket read timeout to incomplete requests
• require exactly one non-empty handshake nonce, capped at 256 characters
• serialize attestation-provider failures through the normal cA2A error path
• document the reference-server bounds and add raw HTTP regression tests

Why

The unauthenticated reference server converted Content-Length with a bare int() and decoded request bodies without handling invalid UTF-8. Malformed traffic could terminate worker handlers outside the structured error path. Body reads also had no timeout, and handshake query/nonce input was not explicitly bounded before invoking an attestation provider.

Impact

Well-formed clients are unchanged. Malformed or ambiguous requests fail promptly and predictably. Production deployments should still apply platform-level connection and rate limits.

Validation

• focused live HTTP tests: 20 passed
• full suite: 398 passed, 3 skipped (hardware-only)
• Ruff check and format gate
• mypy strict
• Bandit
• git diff --check