Deepen protocol_translation behind prepare_exchange for the standard Chat floor #442

Part of #441

Outcome

Deepen the existing src-python/protocol_translation.py seam so the Gateway handler does not keep growing Chat/Responses helpers. Callers see one small interface:

prepare_exchange(request_body, route_attempt, tool_plan) -> PreparedExchange | NonForwardable

Do not create a second Chat adapter module. Do not add protocol adaptation back into codex_proxy.py (ADR-0001).

Required behavior

PreparedExchange hides:

• Responses → Chat request translation
• Chat JSON → Responses
• progressive stream decoding
• a request-scoped tool/call/item identity registry
• the adaptation summary
• fail-closed reasons

The first landable slice is the standard Chat floor, not Collaboration V2:

• progressive text
• multi-turn history
• standard function declaration / call / result
• usage / error / cancellation
• stable IDs
• exactly one terminal
• byte-identical upstream bodies for the same caller body, protocol, capabilities, and ordered tool declarations
• no hidden fallback to Responses, Official, another model, or another Provider

Unknown or irreversible semantics raise UnsupportedProtocolTranslationError (or NonForwardable) before upstream sampling.

Old helpers stay importable from codex_proxy via re-export until callers migrate. No test-import rewrite in this issue.

Acceptance criteria

• Handler / RouteAttempt code paths that need Chat translation go through prepare_exchange (or a documented thin wrapper that only logs Gateway policy).
• protocol_translation.py does not import codex_proxy.
• Standard-floor fixtures in tests/test_chat_completions_gateway.py and the existing routing Chat fixtures pass.
• Same input + capabilities + ordered declarations produce a byte-identical upstream body.
• Unknown fields / items fail closed and cannot disappear.
• Collaboration V2 remains out of scope here and stays on 0.1.8 Beta6: Implement generic Collaboration V2 over Chat Completions #394.

Blocked by

• 0.1.8 Beta6: Define the real Codex-to-Chat runtime-field and V2 item contract #253
• 0.1.8 Beta6: Define progressive Chat and Collaboration V2 streaming #254

Out of scope

• RoutePlan / SSE extraction (Refactor: extract route-plan and SSE helper clusters from codex_proxy.py monolith #413)
• Level-3 DAG cleanup (Cleanup: remove unused Level-3 dynamic-DAG subagent machinery from production path #414)
• V2 tools, aliases, and lifecycle (0.1.8 Beta6: Implement generic Collaboration V2 over Chat Completions #394 / 0.1.8 Beta6: Verify the complete generic Chat Collaboration V2 lifecycle #395)
• Provider-named qualification (0.1.8 Beta6: Complete the GLM-5.2 Ollama Chat and V2 CLI path #285 / 0.1.8 Beta6: Complete the K2.7 Code Ollama Chat and V2 CLI path #286 / 0.1.8 Beta6: Complete the DeepSeek V4 Flash 0731 Chat CLI path #396)
• Splitting tests/test_routing.py