feat(backend): adapt every completion to the detected backend (workspace T-137)

One choke point in the sanitized client: the flavor is probed once
(the boilerroom wrapper names itself on /health, a bare llama-server
serves /props, Ollama answers neither) and every completion adapts.
The agents' tool_choice "required" survives only on Ollama — advisory
there, enforced by llama-server, an unbreakable tool loop through the
wrapper. Through the wrapper every completion carries webber's session
identity: session webber, eviction_order 20 in the decided ranking,
configurable via settings. The wrapper's balancing and compaction
signals are read from the response body's extra fields — an httpx
event-hook variant was tried and never fires under the openai SDK.

The enabling fix: the sanitized client was never in the request path.
The provider assigned self._openai_client, an attribute nobody reads —
OllamaProvider.client serves self._client — so every completion has
bypassed the null-content sanitizer since the class was introduced.
Exposed when the wrapper 503'd a session-less request the choke point
should have named; the client now goes through the constructor's
official openai_client parameter, and a wiring test pins
provider.client to the sanitized type. The same bug exists in tatlock
(its T-6, filed).

Verified against the live wrapper from the dev server: flavor
boilerroom detected, a tool-using explore run answered in 4.8 s with
no tool loop, webber resident at rank 20, and the wrapper parked
librarian and tatlock-experts to seat it — the ranking doing exactly
its job. Six new tests (227 green), five mutation-checked: the rank
default, the strip condition, the session-add condition, the
no-cache-on-failure rule, and the client wiring.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

webber-api/src/shared/config.py

@@ -67,6 +67,13 @@ class Settings(BaseSettings):
     ollama_agent_model: str = "gemma4:e2b"
     ollama_embed_model: str = "nomic-embed-text:latest"
 
+    # Session identity through the boilerroom wrapper (workspace T-137):
+    # rank 20 in the decided ordering — tatlock phases 40, experts 35,
+    # librarian 30, webber 20; lower parks sooner. The fields only mean
+    # something to the wrapper; Ollama and a bare llama-server ignore them.
+    backend_session_name: str = "webber"
+    backend_session_rank: int = 20
+
     # Auth - Tatlock integration
     tatlock_api_url: str | None = "http://tatlock:8000"
     internal_api_key: str | None = None