refactor: gadfly round 2 — both halves of an OpenAI-compat built-in register together

Same class of finding as round 1, one level in: I factored the DSN-scheme half
of the kimi/qwen duplication into openaiCompatScheme and left the eager
provider half copy-pasted, so a third built-in still had six lines to clone —
including both credential rules, which is exactly the pair you do not want
re-typed.

registerOpenAICompatBuiltin now installs both halves from one call. The rules
that matter hold by construction for every future caller: WithAPIKey passed
unconditionally (an unset key must not fall through to OPENAI_API_KEY), and
WithAPIKeyName naming that same variable in the 401 hint. Registering kimi and
qwen is now one line each.

Also fixed a cross-reference the ADR got wrong: Qwen's image-input caveat is
README matrix footnote ⁴, not ³ — ³ is kimi's. I wrote "³, shared with kimi"
in the ADR and then gave Qwen its own footnote in the README.

The break-check harness needed fixing before any of this could be trusted:
three of its mutations targeted lines this refactor moved, so they matched
nothing, the code was never broken, and the suite reported "test still passed"
— identical output to a test that genuinely misses the bug. Mutations are now
verified to have landed (sha before/after) and the suite fails loudly if one
doesn't. Two new cases cover the helper: dropping the unconditional WithAPIKey,
and dropping the scheme-half registration. 8/8 apply and are caught.

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

docs/adr/0027-qwen-builtin.md

@@ -87,7 +87,7 @@ blocks there too), where every OpenAI-compat target would get them.
 - Chat, streaming, tools, structured output, reasoning effort, and cached-token
   accounting all ride the openai client and inherit its fixes.
 - Image *inputs* work at the client level, but only the `qwen-vl-*` /
-  `qwen3-vl-*` models accept them (matrix footnote ³, shared with kimi).
+  `qwen3-vl-*` models accept them (matrix footnote ⁴; ³ is kimi's).
 - Two model-side quirks are Alibaba's, not majordomo's, and are left to the
   caller rather than papered over: thinking is **on by default** on some models
   (e.g. `qwen3.7-plus`), and Qwen3 *open-source* models require streaming when