feat: opt in to client-owned tool loops

[seonghobae]

Summary

Stacked on the current provider-capability branch fix/provider-temperature-capability-negotiation-on-765-v2 so the diff stays limited to the gateway tool-loop contract.

• reuse the existing provider-neutral proxy_completion path for OpenAI Chat and Responses tool loops
• require explicit X-Contextual-Orchestrator-Tool-Loop: v1 opt-in
• preserve the existing fail-closed 422 for ordinary clients
• reject streaming tool loops until a provider-shape-preserving streaming relay exists
• record bounded analytics for the opt-in passthrough paths
• update ADR 0014 and HTTP honesty coverage

Verification

• uv run --with pytest pytest -q — 1,474 passed
• focused passthrough/tool-shape tests — 31 passed
• python3 -m compileall -q contextual_orchestrator/server.py — passed
• git diff --check — passed

No approval, merge, check success, or deployment evidence is manufactured.

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[seonghobae]

Review/fix pass on exact current HEAD 36e3be0bca5f64b7c5150351b2d505ea536a46a4:

• Fixed all three stale __main__ test calls that caused ruff check F821 failures.
• Focused HTTP-honesty/passthrough suite: 51 passed.
• Ruff and git diff --check: passed.
• The branch is stacked on cf4a4501fa5057f89b21cad5033c5925755cd150; no predecessor evidence was reused.

@opencode-agent please review this exact HEAD.

[seonghobae]

Exact-head verification completed on 36e3be0bca5f64b7c5150351b2d505ea536a46a4 (remote re-fetched immediately before reporting):

• Full unit and contract suite: 1474 passed in 626.39s.
• Focused tool-loop/passthrough suite: 51 passed.
• Ruff and git diff --check: passed.
• No predecessor evidence was reused; the stack base remains cf4a4501fa5057f89b21cad5033c5925755cd150.

@opencode-agent please review this exact current HEAD.

[seonghobae]

Current-head proof for 36e3be0bca5f64b7c5150351b2d505ea536a46a4:

• The client-owned tool loop remains explicit: without X-Contextual-Orchestrator-Tool-Loop: v1, tool execution fails closed; the opt-in path preserves bounded request validation and OpenAI tool shape.
• Focused tool-loop suite: 27 passed.
• Full exact-tree suite: 1474 passed in 650.65s.
• Ruff, compileall, and git diff --check: passed.

This PR remains stacked on the temperature contract parent and protected evidence. No self-approval, admin merge, or force-push was used.