fix(mcp): clarify ASCII-only requirement in server name validation error

Fixes bounty issue #1626

The validate_server_name function uses is_ascii_alphanumeric() but the error
message only mentioned 'letters, numbers' without specifying ASCII-only.
Users trying unicode names like 'test-日本語' received a generic error without
understanding why unicode characters were rejected.

Updated the error message to explicitly state 'ASCII letters a-z/A-Z,
digits 0-9' so users understand the limitation.

Author: factorydroid

cortex-cli/src/mcp_cmd.rs

@@ -957,7 +957,7 @@ fn validate_server_name(name: &str) -> Result<()> {
         .all(|c| c.is_ascii_alphanumeric() || c == '-' || c == '_');
 
     if !is_valid_chars {
-        bail!("invalid server name '{name}' (use letters, numbers, '-', '_')");
+        bail!("invalid server name '{name}' (use ASCII letters a-z/A-Z, digits 0-9, '-', or '_')");
     }
 
     // Must start with a letter or underscore