fix: remove ANTHROPIC_REASONING_MODEL to decouple thinking from model selection (#2081)

ANTHROPIC_REASONING_MODEL was a non-official env var that forced all
requests with thinking params to use a single "reasoning model",
overriding the user's /model selection. Since new Claude Code versions
send adaptive thinking by default, this caused /model to silently fail.

- Remove reasoning_model field and has_thinking_enabled() from model_mapper
- Simplify map_model() to pure type-based matching (haiku/sonnet/opus)
- Remove reasoning model UI field from provider form
- Retain ANTHROPIC_REASONING_MODEL in ENV_EXCLUDES and override-key
  cleanup lists so legacy configs don't leak into common config

src/components/providers/forms/ProviderForm.tsx

@@ -314,7 +314,6 @@ export function ProviderForm({
 
   const {
     claudeModel,
-    reasoningModel,
     defaultHaikuModel,
     defaultSonnetModel,
     defaultOpusModel,
@@ -1567,7 +1566,6 @@ export function ProviderForm({
               onAutoSelectChange={setEndpointAutoSelect}
               shouldShowModelSelector={category !== "official"}
               claudeModel={claudeModel}
-              reasoningModel={reasoningModel}
               defaultHaikuModel={defaultHaikuModel}
               defaultSonnetModel={defaultSonnetModel}
               defaultOpusModel={defaultOpusModel}