refactor(model-routing): centralize explicit foreground fallback policy (#6020)

* refactor(model-routing): centralize explicit foreground fallback policy

Make foreground fallback an explicit per-user, availability-only policy shared by streaming Chat, non-stream Chat, and Agent runs.

Preserve strict defaults, owner/model and credential boundaries, pinned Agent routes, and truthful per-round provenance/accounting. Carry provider-reported model identifiers through native streaming adapters, non-stream responses, and caches, and keep legacy default_model_fallbacks as tombstoned raw storage that generic settings APIs and agent tools cannot expose or mutate.

* fix(agent-loop): restore rebase-dropped qwen routing, workspace prompt, and temperature clamp

* fix(model-routing): thread selected endpoint identity, fix cost classification and fallback eligibility

* fix(chat): restore stream helpers and harden run stop lifecycle

* fix(model-routing): let numeric provider codes win over symbolic rate-limit statuses

* fix(agent-loop): apply qwen temperature and notes-tool clamps per fallback candidate

* fix(chat): honor queued stop across resend and reload canonical terminal on EOF

* fix(chat): track stop queue and cleanup ownership by per-send generation

* fix(agent-loop): preserve requested temperature for non-qwen fallback candidates

* fix(chat): reserve send ownership before any await and scope stop to the current send

* fix(chat): clear the previous run identity at send reservation

---------

Co-authored-by: RaresKeY <158580472+RaresKeY@users.noreply.github.com>
Co-authored-by: StressTestor <212606152+StressTestor@users.noreply.github.com>

mcp_servers/email_server.py

@@ -1802,7 +1802,6 @@ async def _ai_draft_reply_to_email(uid, folder="INBOX", reply_all=False, account
         from src.endpoint_resolver import (
             resolve_endpoint,
             resolve_utility_fallback_candidates,
-            resolve_chat_fallback_candidates,
         )
         from src.llm_core import llm_call_async_with_fallback
     except Exception as exc:
@@ -1843,13 +1842,6 @@ async def _ai_draft_reply_to_email(uid, folder="INBOX", reply_all=False, account
         utility_fallbacks = resolve_utility_fallback_candidates() or []
     for cand in utility_fallbacks:
         _add(*cand)
-    try:
-        chat_fallbacks = resolve_chat_fallback_candidates(owner=None) or []
-    except TypeError:
-        chat_fallbacks = resolve_chat_fallback_candidates() or []
-    for cand in chat_fallbacks:
-        _add(*cand)
-
     if not candidates:
         return {"error": "No LLM endpoint configured for AI reply"}