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

@@ -2,8 +2,8 @@
 
 use anyhow::{Context, Result, bail};
 use clap::{ArgGroup, Parser};
-use cortex_common::dirs::get_cortex_home;
 use cortex_common::CliConfigOverrides;
+use cortex_common::dirs::get_cortex_home;
 use std::io::{self, BufRead, Write};
 
 // ============================================================================
@@ -918,7 +918,7 @@ 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