refactor: gadfly round 3 — one table owns the OpenAI-compat contract

Three findings, and the first two are the same recurring shape.

envKeyForProvider (env.go) is now the single definition of the LLM_<NAME>
form. It lived in two places — lazy resolution in registry.go and the
missing-key hint in openaiCompatScheme — with a comment on the second asserting
it matched the first. A comment is not enforcement: if either had drifted, a
keyless DSN target would have named a variable that does nothing, and nothing
would have failed.

The kimi and qwen test files had become near-identical, which is round 1's
finding at the level above it: I deduped the fixtures, then left two parallel
suites asserting the same four things. They are now ONE table in
builtin_openaicompat_test.go — endpoint + credential, missing key fails closed
naming its own variable and never reaching the network, the name:// DSN
reaching another host, and a keyless DSN naming LLM_<NAME> instead of the
built-in's key. Adding an OpenAI-compat built-in is a table row that
immediately owes all four; builtin_kimi_test.go is deleted because the table
covers it. Only genuinely qwen-specific tests remain in the qwen file: the
reverse credential leak and the reasoning_effort wire claim ADR-0027 rests on.

Also trimmed ProviderQwen's doc comment, which restated the ADR-0027 rationale
already given at the registration site.

The break-check suite caught its own rot again — two mutations went stale when
these tests were renamed, and the landed-check reported them loudly instead of
passing them off as green. Now 9 cases, including one that drifts
envKeyForProvider to prove the shared helper is load-bearing. 9/9 apply and are
caught.

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

registry.go

@@ -263,7 +263,7 @@ func (r *Registry) providerFor(name string) (llm.Provider, error) {
 		return nil, envErr
 	}
 
-	envKey := "LLM_" + strings.ToUpper(strings.ReplaceAll(name, "-", "_"))
+	envKey := envKeyForProvider(name)
 	envVal := r.envLookup(envKey)
 	if envVal == "" {
 		return nil, fmt.Errorf("%w: %q (checked registry and %s env var)", ErrUnknownProvider, name, envKey)