Add example servers for SEP-2053 variant patterns (#4)

Add four new example servers demonstrating different SEP-2053 variant
patterns:

- model-optimized (same tools, different descriptions per LLM family)
- trading (API versioning with deprecation lifecycle)
- research (context budget management)
- Refactor the existing variants-http example into a GitHub-style server
with custom ranking.

Signed-off-by: Kurt Degiorgio <kdegiorgio@bloomberg.net>

go/sdk/examples/server/model-optimized/README.md

@@ -0,0 +1,118 @@
+# Model-Optimized Weather Service
+
+A variant-aware MCP server that exposes the same tools (`get_weather`, `get_forecast`) with different descriptions optimized for different LLM families. This is the primary use case described in SEP-2053.
+
+**Pattern demonstrated:** Same tools, different descriptions per variant.
+
+## Variants
+
+| Variant | Description Style | Target |
+|---|---|---|
+| `claude-optimized` | Verbose, structured with explicit usage guidance | Anthropic Claude models |
+| `gpt-optimized` | Concise, function-style with JSON Schema emphasis | OpenAI GPT models |
+| `compact` | Minimal, bare-minimum descriptions | Context-constrained environments |
+
+All three variants expose the same two tools with the same behavior — only the tool descriptions differ.
+
+## Custom Ranking
+
+Clients send a `"modelFamily"` hint (e.g., `"anthropic"`, `"openai"`). The ranking function matches the hint to the variant's `modelFamily`, falling back to priority order.
+
+## Run
+
+```bash
+go run ./examples/server/model-optimized
+```
+The server listens on `http://localhost:8080`.
+
+Connect any MCP client to `http://localhost:8080`.
+
+## Try It
+
+### 1. Initialize without hints (default ranking)
+
+```bash
+curl -s -X POST http://localhost:8080/mcp \
+  -H 'Content-Type: application/json' \
+  -d '{
+    "jsonrpc": "2.0", "id": 1,
+    "method": "initialize",
+    "params": {
+      "protocolVersion": "2025-03-26",
+      "clientInfo": {"name": "test", "version": "1.0"},
+      "capabilities": {}
+    }
+  }'
+```
+
+The first variant in the response is the **default** — clients that don't specify a variant in subsequent requests will use it. Without hints, `claude-optimized` is the default:
+
+```json
+{
+  "availableVariants": [
+    {
+      "id": "claude-optimized",
+      "hints": { "contextSize": "verbose", "modelFamily": "anthropic" },
+      "status": "stable"
+    },
+    {
+      "id": "gpt-optimized",
+      "hints": { "contextSize": "standard", "modelFamily": "openai" },
+      "status": "stable"
+    },
+    {
+      "id": "compact",
+      "hints": { "contextSize": "compact", "modelFamily": "any" },
+      "status": "stable"
+    }
+  ]
+}
+```
+
+### 2. Initialize with an OpenAI hint (ranking changes)
+
+```bash
+curl -s -X POST http://localhost:8080/mcp \
+  -H 'Content-Type: application/json' \
+  -d '{
+    "jsonrpc": "2.0", "id": 1,
+    "method": "initialize",
+    "params": {
+      "protocolVersion": "2025-03-26",
+      "clientInfo": {"name": "test", "version": "1.0"},
+      "capabilities": {
+        "experimental": {
+          "io.modelcontextprotocol/server-variants": {
+            "variantHints": {
+              "hints": {"modelFamily": "openai"}
+            }
+          }
+        }
+      }
+    }
+  }'
+```
+
+Now `gpt-optimized` is ranked first and becomes the default:
+
+```json
+{
+  "availableVariants": [
+    {
+      "id": "gpt-optimized",
+      "hints": { "contextSize": "standard", "modelFamily": "openai" },
+      "status": "stable"
+    },
+    {
+      "id": "claude-optimized",
+      "hints": { "contextSize": "verbose", "modelFamily": "anthropic" },
+      "status": "stable"
+    },
+    {
+      "id": "compact",
+      "hints": { "contextSize": "compact", "modelFamily": "any" },
+      "status": "stable"
+    }
+  ]
+}
+```