chore: Release server.json schema version 2025-12-11 (#841)

Prepared by Claude Code, iterated and reviewed by me.

## Summary

This PR releases the server.json schema version `2025-12-11`,
incorporating URL template variables for remote servers from #570.

### What's New

- **URL Template Variables for Remote Servers** - Remote servers can now
define configurable variables in URLs using `{curly_braces}` notation,
enabling multi-tenant deployments.

### Schema Version
- `2025-10-17` → `2025-12-11`

## Release Sequence

1. ~~**First: Merge static repo PR** -
https://github.com/modelcontextprotocol/static/pull/11 (publishes schema
to versioned URL)~~
2. **Then: Merge this PR** - Updates registry code to reference the new
schema version

## Changes in This PR

- [x] Update CHANGELOG.md with release notes
- [x] Update `CurrentSchemaVersion` in `pkg/model/constants.go`
- [x] Update schema example URL in `docs/reference/api/openapi.yaml`
- [x] Update schema URL in `pkg/api/v0/types.go`
- [x] Update all documentation examples to use new schema version
- [x] Regenerate `server.schema.json`

## Test Plan
- [x] `make check` passes (lint, schema validation, example validation)
- [x] Unit tests pass for validators and publisher commands

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

docs/modelcontextprotocol-io/remote-servers.mdx

@@ -7,7 +7,7 @@ The MCP Registry supports remote MCP servers via the `remotes` property in `serv
 
 ```json server.json highlight={7-12}
 {
-  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-10-17/server.schema.json",
+  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-12-11/server.schema.json",
   "name": "com.example/acme-analytics",
   "title": "ACME Analytics",
   "description": "Real-time business intelligence and reporting platform",
@@ -31,7 +31,7 @@ Specify the transport by setting the `type` property of the `remotes` entry to e
 
 ```json server.json highlight={9,13}
 {
-  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-10-17/server.schema.json",
+  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-12-11/server.schema.json",
   "name": "com.example/acme-analytics",
   "title": "ACME Analytics",
   "description": "Real-time business intelligence and reporting platform",
@@ -49,13 +49,71 @@ Specify the transport by setting the `type` property of the `remotes` entry to e
 }
 ```
 
+## URL Template Variables
+
+Remote servers can define URL template variables using `{curly_braces}` notation. This enables multi-tenant deployments where a single server definition can support multiple endpoints with configurable values:
+
+```json server.json highlight={10-17}
+{
+  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-12-11/server.schema.json",
+  "name": "com.example/acme-analytics",
+  "title": "ACME Analytics",
+  "description": "Real-time business intelligence and reporting platform",
+  "version": "2.0.0",
+  "remotes": [
+    {
+      "type": "streamable-http",
+      "url": "https://{tenant_id}.analytics.example.com/mcp",
+      "variables": {
+        "tenant_id": {
+          "description": "Your tenant identifier (e.g., 'us-cell1', 'emea-cell1')",
+          "isRequired": true
+        }
+      }
+    }
+  ]
+}
+```
+
+When configuring this server, users provide their `tenant_id` value, and the URL template gets resolved to the appropriate endpoint (e.g., `https://us-cell1.analytics.example.com/mcp`).
+
+Variables support additional properties like `default`, `choices`, and `isSecret`:
+
+```json server.json highlight={12-22}
+{
+  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-12-11/server.schema.json",
+  "name": "com.example/multi-region-mcp",
+  "title": "Multi-Region MCP",
+  "description": "MCP server with regional endpoints",
+  "version": "1.0.0",
+  "remotes": [
+    {
+      "type": "streamable-http",
+      "url": "https://api.example.com/{region}/mcp",
+      "variables": {
+        "region": {
+          "description": "Deployment region",
+          "isRequired": true,
+          "choices": [
+            "us-east-1",
+            "eu-west-1",
+            "ap-southeast-1"
+          ],
+          "default": "us-east-1"
+        }
+      }
+    }
+  ]
+}
+```
+
 ## HTTP Headers
 
 MCP clients can be instructed to send specific HTTP headers by adding the `headers` property to the `remotes` entry:
 
 ```json server.json highlight={11-18}
 {
-  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-10-17/server.schema.json",
+  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-12-11/server.schema.json",
   "name": "com.example/acme-analytics",
   "title": "ACME Analytics",
   "description": "Real-time business intelligence and reporting platform",
@@ -83,7 +141,7 @@ The `remotes` property can coexist with the `packages` property in `server.json`
 
 ```json server.json highlight={7-22}
 {
-  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-10-17/server.schema.json",
+  "$schema": "https://static.modelcontextprotocol.io/schemas/2025-12-11/server.schema.json",
   "name": "io.github.username/email-integration-mcp",
   "title": "Email Integration",
   "description": "Send emails and manage email accounts",