fix(decisioning): project malformed-JSON 2xx responses to AdcpError

[bokelley]

Refs #453

When an upstream returns a successful (2xx) status with a non-JSON body (e.g. a CDN or proxy returning an HTML error page), UpstreamHttpClient._request previously propagated a raw json.JSONDecodeError from response.json(). Adopters catch AdcpError throughout; an unhandled ValueError subclass breaks that contract and is invisible in typed code. This PR wraps the call in except ValueError and re-raises as AdcpError(SERVICE_UNAVAILABLE, recovery="transient") — the same code used for 5xx responses, since an upstream returning HTML for a JSON endpoint is indistinguishable from a transient infrastructure failure.

This is a re-cut of the original branch from #767 (claude/issue-453-json-decode-error-handling), which was behind main and had maintainer edits disabled. The diff is identical to #767 but rebased cleanly on current main.

What changed

• src/adcp/decisioning/upstream.py: wrap response.json() in try/except ValueError; raise AdcpError(SERVICE_UNAVAILABLE) with recovery="transient" and from exc chain.
• tests/test_upstream_helpers.py: add test_200_with_malformed_json_raises_service_unavailable — mocks a 200 with an HTML body and asserts code, recovery, and __cause__.

What tested

• pytest tests/test_upstream_helpers.py -q — 35 passed, 0 failed
• mypy src/adcp/decisioning/upstream.py — clean
• ruff check src/adcp/decisioning/upstream.py tests/test_upstream_helpers.py — clean

Pre-PR review

• code-reviewer: approved — flagged missing @pytest.mark.asyncio (false positive: asyncio_mode = "auto" in pyproject.toml); one nit on truncating exc in message (low severity, AdcpError is internal)
• dx-expert: approved — SERVICE_UNAVAILABLE / transient is correct for CDN/proxy interception; nit on adding bounded response.text[:200] snippet to message; no blockers

Nits (not fixed):

• Error message embeds raw exc text rather than a body-length hint; useful for debugging, acceptable for an internal error type
• recovery="transient" is correct per the spec but can't distinguish "CDN blip" from "permanently misconfigured upstream" — no better enum value exists

Triage-managed PR. This bot does not currently iterate on
review comments or PR conversation threads (only on the source
issue). To unblock:

• Push fixup commits directly: gh pr checkout <num> →
  fix → push.
• Or re-trigger: comment /triage execute on the source
  issue.

See adcp#3121
for context.

Session: https://claude.ai/code/session_018wq8sxhkG9c2WW4vgpYgS8

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