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>

data/seed.json

@@ -1,6 +1,6 @@
 [
   {
-    "$schema": "https://static.modelcontextprotocol.io/schemas/2025-10-17/server.schema.json",
+    "$schema": "https://static.modelcontextprotocol.io/schemas/2025-12-11/server.schema.json",
     "name": "io.github.domdomegg/airtable-mcp-server",
     "description": "Read and write access to Airtable database schemas, tables, and records.",
     "repository": {
@@ -62,7 +62,7 @@
     ]
   },
   {
-    "$schema": "https://static.modelcontextprotocol.io/schemas/2025-10-17/server.schema.json",
+    "$schema": "https://static.modelcontextprotocol.io/schemas/2025-12-11/server.schema.json",
     "name": "io.github.domdomegg/airtable-mcp-server",
     "description": "Read and write access to Airtable database schemas, tables, and records.",
     "repository": {
@@ -115,7 +115,7 @@
     ]
   },
   {
-    "$schema": "https://static.modelcontextprotocol.io/schemas/2025-10-17/server.schema.json",
+    "$schema": "https://static.modelcontextprotocol.io/schemas/2025-12-11/server.schema.json",
     "name": "io.github.domdomegg/time-mcp-nuget",
     "description": "Get the current UTC time in RFC 3339 format.",
     "repository": {
@@ -136,7 +136,7 @@
     ]
   },
   {
-    "$schema": "https://static.modelcontextprotocol.io/schemas/2025-10-17/server.schema.json",
+    "$schema": "https://static.modelcontextprotocol.io/schemas/2025-12-11/server.schema.json",
     "name": "io.github.domdomegg/time-mcp-pypi",
     "description": "Get the current UTC time in RFC 3339 format.",
     "repository": {