[Fix] Classify SessionServer endpoints and fail closed on trace errors

[matrix72c]

Summary

SessionServer is a catch-all HTTP proxy, but only two requests represent model generations: POST /v1/messages and POST /v1/chat/completions.

This change makes endpoint policy explicit before request/response processing and makes trace-enabled generations fail closed when the response cannot be recorded. Auxiliary API calls remain transparent, while the client-visible result stays consistent with the training trace stored by SessionServer.

The response-reliability changes previously proposed in #2059 are consolidated here so endpoint and trace policy are reviewed in one PR.

Problem

The proxy cannot safely infer the endpoint from the JSON body. For example, POST /v1/messages/count_tokens contains messages but is not a model generation. Treating it as one can invoke on_request, inject generation-only fields, filter tools, or invoke on_response unexpectedly.

For trace-enabled generations, malformed JSON, non-object response bodies, incomplete SSE streams, downstream disconnects, or a failing on_response hook can leave the trace store without a complete assistant turn. Returning a normal success response in that situation allows the caller to observe a turn that cannot be used for training.

Implementation

• Add a small private _is_generation_endpoint(method, path) predicate. It uses the normalized method and path, ignores query strings, and recognizes only POST /v1/messages and POST /v1/chat/completions as generation endpoints.
• Keep format detection independent from generation classification, so /v1/messages/count_tokens, /v1/messages/batches, and future /v1/messages/... paths still receive the correct Anthropic headers.
• Keep endpoint policy explicit and local to _handle_request; no ContextVar, wrapper handler, shared enum, or cross-project abstraction is introduced.
• For non-generation JSON object requests, remove only the top-level SessionServer-owned session_id. Non-object bodies and objects without session_id remain byte-for-byte unchanged whenever possible.
• Derive response tracing from both the endpoint policy and the upstream status code. Non-2xx responses never enter the cleaner or trace hooks, and explicit upstream error envelopes are passed through without being wrapped again.
• For trace-enabled streams, retain the original upstream chunks and delay [DONE] until parsing and on_response complete. If parsing or the response hook fails, return a native 500 error or SSE error event without [DONE].
• Continue draining the upstream stream after a downstream connection reset so a complete trace can still be collected.
• Handle stream preparation and connection-reset behavior directly in the request handler instead of adding a one-use helper.

Successful generation behavior, the public SessionServer API, and the existing /v1/responses 501 rejection remain unchanged. No LMDeploy code or management endpoint implementation is changed, and fix/chunk-loss-detached-head-memory is untouched.

Tests

Added focused aiohttp fake-upstream coverage for:

• generation and non-generation method/path classification;
• transparent count_tokens and auxiliary endpoint forwarding with no generation hooks;
• successful training responses and delayed stream completion;
• evaluation requests that skip on_response;
• upstream JSON/SSE errors and all non-2xx responses remaining unchanged;
• malformed, non-object, incomplete, and untraceable 2xx responses failing closed;
• response-hook failures producing native errors without [DONE]; and
• downstream disconnects before headers and during streaming while the upstream is still drained and traced.

Validation performed:

Focused endpoint/trace suite: 25 passed
ruff check: All checks passed
python -m py_compile: passed
git diff --check: passed