tools: 清理 fetch_url.go S1017 lint 警告

matchAllowedHost 里两处 if strings.HasPrefix(...) pattern = pattern[N:]
替成无条件 strings.TrimPrefix, 行为完全等价 (前缀不匹配时
TrimPrefix 是 no-op). 触发 S1017 (Should replace this if statement
with an unconditional strings.TrimPrefix).

跟 475e353 在 httpclient/ssrf.go 上做的清理一模一样, 当时
只清了一处, fetch_url.go 里 copy 了同款 pattern 漏了.

净 -4 行, 函数语义不变, 现有测试 (fetch_url_test.go 覆盖的精确
/ 后缀 / wildcard 三种 host 匹配路径) 不变.

Co-Authored-By: Claude <noreply@anthropic.com>

internal/mcp/tools/fetch_url.go

@@ -209,12 +209,8 @@ func hostAllowed(host string, allowed []string) bool {
 		if pattern == host {
 			return true
 		}
-		if strings.HasPrefix(pattern, "*.") {
-			pattern = pattern[2:]
-		}
-		if strings.HasPrefix(pattern, ".") {
-			pattern = pattern[1:]
-		}
+		pattern = strings.TrimPrefix(pattern, "*.")
+		pattern = strings.TrimPrefix(pattern, ".")
 		if host == pattern || strings.HasSuffix(host, "."+pattern) {
 			return true
 		}