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-tauri/src/services/provider/mod.rs

@@ -1734,9 +1734,9 @@ impl ProviderService {
             // Auth
             "ANTHROPIC_API_KEY",
             "ANTHROPIC_AUTH_TOKEN",
-            // Models (5 fields)
+            // Models (4 fields + 1 legacy)
             "ANTHROPIC_MODEL",
-            "ANTHROPIC_REASONING_MODEL",
+            "ANTHROPIC_REASONING_MODEL", // legacy: 已废弃，但旧配置可能残留
             "ANTHROPIC_DEFAULT_HAIKU_MODEL",
             "ANTHROPIC_DEFAULT_OPUS_MODEL",
             "ANTHROPIC_DEFAULT_SONNET_MODEL",