feat: implement DNS namespace validation and remote URL uniqueness checks (#306)

## Summary

- Add reverse-DNS namespace format validation for server names
(namespace/name)
- Validate remote URLs match the DNS namespace derived from hostname  
- Prevent duplicate remote URLs across different servers
- Support localhost/development URLs without namespace restrictions
- Add comprehensive test coverage for validation logic
- Update database layer to support remote_url filtering

Fixes #180 by ensuring proper namespace governance and preventing URL
conflicts.

---------

Co-authored-by: Claude <noreply@anthropic.com>

docs/server-json/examples.md

@@ -186,7 +186,7 @@ This will essentially instruct the MCP client to execute `dnx Knapcode.SampleMcp
 ```json
 {
   "server": {
-    "name": "com.example/mcp-fs",
+    "name": "io.modelcontextprotocol.anonymous/mcp-fs",
     "description": "Cloud-hosted MCP filesystem server",
     "repository": {
       "url": "https://github.com/example/remote-fs",
@@ -199,7 +199,7 @@ This will essentially instruct the MCP client to execute `dnx Knapcode.SampleMcp
     "remotes": [
       {
         "transport_type": "sse",
-        "url": "http://mcp-fs.example.com/sse"
+        "url": "http://mcp-fs.anonymous.modelcontextprotocol.io/sse"
       }
     ]
   },
@@ -418,7 +418,7 @@ The `dnx` tool ships with the .NET 10 SDK, starting with Preview 6.
 ```json
 {
   "server": {
-    "name": "com.example/hybrid-mcp",
+    "name": "io.modelcontextprotocol.anonymous/hybrid-mcp",
     "description": "MCP server available as both local package and remote service",
     "repository": {
       "url": "https://github.com/example/hybrid-mcp",
@@ -448,7 +448,7 @@ The `dnx` tool ships with the .NET 10 SDK, starting with Preview 6.
     "remotes": [
       {
         "transport_type": "sse",
-        "url": "https://mcp.example.com/sse",
+        "url": "https://mcp.anonymous.modelcontextprotocol.io/sse",
         "headers": [
           {
             "name": "X-API-Key",
@@ -466,7 +466,7 @@ The `dnx` tool ships with the .NET 10 SDK, starting with Preview 6.
       },
       {
         "transport_type": "streamable",
-        "url": "https://mcp.example.com/http"
+        "url": "https://mcp.anonymous.modelcontextprotocol.io/http"
       }
     ]
   },