Fix _meta field format for MCP spec compliance (#367)

## Summary

Updates the `_meta` field structure to be compliant with the MCP
specification by using proper `prefix/name` format for vendor
extensions.

### Changes

- **Registry metadata**: `io.modelcontextprotocol.registry` →
`io.modelcontextprotocol.registry/official`
- **Publisher metadata**: `publisher` →
`io.modelcontextprotocol.registry/publisher-provided`
- **Database migration**: Updates indexes to use new field paths
- **Updated all code**: Service layer, validators, tests, and seed data

### Format Before
```json
{
  "_meta": {
    "publisher": { "license": "MIT" },
    "io.modelcontextprotocol.registry": { "id": "...", "is_latest": true }
  }
}
```

### Format After
```json
{
  "_meta": {
    "io.modelcontextprotocol.registry/publisher-provided": { "license": "MIT" },
    "io.modelcontextprotocol.registry/official": { "id": "...", "is_latest": true }
  }
}
```

This resolves the naming collision with the planned top-level
`publisher` field and ensures compliance with MCP specification
requirements.

Fixes #356

docs/reference/server-json/generic-server-json.md

@@ -53,7 +53,7 @@ The official registry has some more restrictions on top of this. See the [offici
     }
   ],
   "_meta": {
-    "publisher": {
+    "io.modelcontextprotocol.registry/publisher-provided": {
       "tool": "npm-publisher",
       "version": "1.0.1",
       "build_info": {
@@ -94,7 +94,7 @@ For MCP servers located within a subdirectory of a larger repository (monorepo s
     }
   ],
   "_meta": {
-    "publisher": {
+    "io.modelcontextprotocol.registry/publisher-provided": {
       "tool": "npm-publisher",
       "version": "1.0.1",
       "build_info": {
@@ -138,7 +138,7 @@ Suppose your MCP server application requires a `mcp start` CLI arguments to star
     }
   ],
   "_meta": {
-    "publisher": {
+    "io.modelcontextprotocol.registry/publisher-provided": {
       "tool": "nuget-publisher",
       "version": "2.1.0",
       "build_info": {
@@ -242,7 +242,7 @@ This will essentially instruct the MCP client to execute `dnx Knapcode.SampleMcp
     }
   ],
   "_meta": {
-    "publisher": {
+    "io.modelcontextprotocol.registry/publisher-provided": {
       "tool": "ci-publisher",
       "version": "3.2.1",
       "build_info": {
@@ -277,7 +277,7 @@ This will essentially instruct the MCP client to execute `dnx Knapcode.SampleMcp
     }
   ],
   "_meta": {
-    "publisher": {
+    "io.modelcontextprotocol.registry/publisher-provided": {
       "tool": "cloud-deployer",
       "version": "2.4.0",
       "build_info": {
@@ -331,7 +331,7 @@ This will essentially instruct the MCP client to execute `dnx Knapcode.SampleMcp
     }
   ],
   "_meta": {
-    "publisher": {
+    "io.modelcontextprotocol.registry/publisher-provided": {
       "tool": "poetry-publisher",
       "version": "1.8.3",
       "build_info": {
@@ -382,7 +382,7 @@ The `dnx` tool ships with the .NET 10 SDK, starting with Preview 6.
     }
   ],
   "_meta": {
-    "publisher": {
+    "io.modelcontextprotocol.registry/publisher-provided": {
       "tool": "dotnet-publisher",
       "version": "8.0.100",
       "build_info": {
@@ -494,7 +494,7 @@ The `dnx` tool ships with the .NET 10 SDK, starting with Preview 6.
     }
   ],
   "_meta": {
-    "publisher": {
+    "io.modelcontextprotocol.registry/publisher-provided": {
       "tool": "docker-buildx",
       "version": "0.12.1",
       "build_info": {
@@ -578,7 +578,7 @@ The `dnx` tool ships with the .NET 10 SDK, starting with Preview 6.
     }
   ],
   "_meta": {
-    "publisher": {
+    "io.modelcontextprotocol.registry/publisher-provided": {
       "tool": "hybrid-deployer",
       "version": "1.7.2",
       "build_info": {
@@ -622,7 +622,7 @@ The `dnx` tool ships with the .NET 10 SDK, starting with Preview 6.
     }
   ],
   "_meta": {
-    "publisher": {
+    "io.modelcontextprotocol.registry/publisher-provided": {
       "tool": "mcpb-publisher",
       "version": "1.0.0",
       "build_info": {
@@ -711,7 +711,7 @@ Some CLI tools bundle an MCP server, without a standalone MCP package or a publi
     }
   ],
   "_meta": {
-    "publisher": {
+    "io.modelcontextprotocol.registry/publisher-provided": {
       "tool": "legacy-publisher",
       "version": "0.8.1",
       "build_info": {

docs/reference/server-json/server.schema.json

@@ -453,12 +453,12 @@
               "description": "Extension metadata using reverse DNS namespacing for vendor-specific data",
               "additionalProperties": true,
               "properties": {
-                "publisher": {
+                "io.modelcontextprotocol.registry/publisher-provided": {
                   "type": "object",
                   "description": "Publisher-provided metadata for downstream registries",
                   "additionalProperties": true
                 },
-                "io.modelcontextprotocol.registry": {
+                "io.modelcontextprotocol.registry/official": {
                   "type": "object",
                   "description": "Official MCP registry metadata (read-only, added by registry)",
                   "additionalProperties": true