feat(qwen): Alibaba Qwen built-in over Model Studio's OpenAI-compatible mode

Adds the `qwen` built-in provider and the `qwen://` DSN scheme, keyed by
QWEN_API_KEY and defaulting to Model Studio's international host. Like kimi
(ADR-0026) it is `provider/openai` pointed elsewhere — no new client.

Model Studio serves the same models over two protocols, so the real decision
was which wire format to speak. ADR-0027 records why it is the OpenAI one:
down the anthropic client `ReasoningEffort` is ignored by design, structured
output rides the first-party `output_config.format` mechanism the shim does
not implement, and cached-token accounting reads Anthropic-only usage fields.
Each of those fails silently rather than loudly, which is what makes the
choice worth writing down. The shim stays reachable ad hoc via an
`anthropic://` DSN.

The kimi and qwen DSN factories were byte-identical, so they now share one
`openaiCompatScheme` helper: the "credential comes from the DSN token, and
the missing-key hint names LLM_<NAME>" rules hold by construction instead of
by copy.

Tests are hermetic and break-checked (all six fail on a deliberate mutation),
including the reverse credential leak — a visible QWEN_API_KEY must not
authenticate the openai built-in — and reasoning_effort asserted on the wire
body, which is the ADR's load-bearing claim.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

.env.example

@@ -7,6 +7,7 @@ OLLAMA_API_KEY=your-ollama-cloud-key-here
 # Built-in provider keys (each optional; only needed for the providers you use).
 #OPENAI_API_KEY=sk-...
 #KIMI_API_KEY=sk-...           # Moonshot AI (Kimi); provider name "kimi"
+#QWEN_API_KEY=sk-...           # Alibaba Model Studio (Qwen); provider name "qwen"
 #ANTHROPIC_API_KEY=sk-ant-...
 #GOOGLE_API_KEY=...