fix(mcp): correct Codex MCP configuration format to [mcp_servers]

BREAKING CHANGE: The [mcp.servers] format was completely incorrect and not
any official Codex format. The only correct format is [mcp_servers] at the
top level of config.toml.

Changes:
- Remove incorrect [mcp.servers] nested table support
- Always use [mcp_servers] top-level table (official Codex format)
- Auto-migrate and cleanup erroneous [mcp.servers] entries on write
- Preserve error-tolerant import for migrating old incorrect configs
- Simplify sync logic by removing format selection branches (~60 lines)
- Update all documentation and tests to reflect correct format
- Add warning logs when detecting and cleaning incorrect format

Backend (Rust):
- mcp.rs: Simplify sync_enabled_to_codex by removing Target enum
- mcp.rs: sync_single_server_to_codex now always uses [mcp_servers]
- mcp.rs: remove_server_from_codex cleans both locations
- mcp.rs: Update import_from_codex comments to clarify format status
- tests: Rename test to sync_enabled_to_codex_migrates_erroneous_*
- tests: Update assertions to verify migration behavior

Frontend (TypeScript):
- tomlUtils.ts: Prioritize [mcp_servers] format in parsing
- tomlUtils.ts: Update error messages to guide correct format

Documentation:
- README.md: Correct MCP format reference to [mcp_servers]
- CLAUDE.md: Add comprehensive format specification with examples

All 79 tests pass. This ensures backward compatibility while enforcing
the correct Codex official standard going forward.

Refs: https://github.com/openai/codex/issues/3441

src-tauri/src/claude_mcp.rs

@@ -139,13 +139,11 @@ pub fn upsert_mcp_server(id: &str, spec: Value) -> Result<bool, AppError> {
     if is_http || is_sse {
         let url = spec.get("url").and_then(|x| x.as_str()).unwrap_or("");
         if url.is_empty() {
-            return Err(AppError::McpValidation(
-                if is_http {
-                    "http 类型的 MCP 服务器缺少 url 字段".into()
-                } else {
-                    "sse 类型的 MCP 服务器缺少 url 字段".into()
-                },
-            ));
+            return Err(AppError::McpValidation(if is_http {
+                "http 类型的 MCP 服务器缺少 url 字段".into()
+            } else {
+                "sse 类型的 MCP 服务器缺少 url 字段".into()
+            }));
         }
     }