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/tools.go

@@ -0,0 +1,91 @@
+package main
+
+import (
+	"context"
+
+	"github.com/modelcontextprotocol/go-sdk/mcp"
+)
+
+type GetWeatherInput struct {
+	Location string `json:"location" jsonschema:"location name or coordinates (e.g. 'San Francisco, CA' or '37.7749,-122.4194')"`
+	Units    string `json:"units,omitempty" jsonschema:"temperature units: celsius, fahrenheit (default: celsius)"`
+}
+
+type GetWeatherOutput struct {
+	Location    string  `json:"location"`
+	Temperature float64 `json:"temperature"`
+	Units       string  `json:"units"`
+	Condition   string  `json:"condition"`
+	Humidity    int     `json:"humidity"`
+	WindSpeed   float64 `json:"windSpeed"`
+}
+
+type GetForecastInput struct {
+	Location string `json:"location" jsonschema:"location name or coordinates"`
+	Days     int    `json:"days,omitempty" jsonschema:"number of days to forecast (1-7, default: 3)"`
+	Units    string `json:"units,omitempty" jsonschema:"temperature units: celsius, fahrenheit (default: celsius)"`
+}
+
+type ForecastDay struct {
+	Date      string  `json:"date"`
+	High      float64 `json:"high"`
+	Low       float64 `json:"low"`
+	Condition string  `json:"condition"`
+}
+
+type GetForecastOutput struct {
+	Location string        `json:"location"`
+	Units    string        `json:"units"`
+	Forecast []ForecastDay `json:"forecast"`
+}
+
+func getWeather(_ context.Context, _ *mcp.CallToolRequest, in GetWeatherInput) (*mcp.CallToolResult, GetWeatherOutput, error) {
+	units := in.Units
+	if units == "" {
+		units = "celsius"
+	}
+	temp := 22.5
+	if units == "fahrenheit" {
+		temp = 72.5
+	}
+	return nil, GetWeatherOutput{
+		Location:    in.Location,
+		Temperature: temp,
+		Units:       units,
+		Condition:   "partly cloudy",
+		Humidity:    65,
+		WindSpeed:   12.3,
+	}, nil
+}
+
+func getForecast(_ context.Context, _ *mcp.CallToolRequest, in GetForecastInput) (*mcp.CallToolResult, GetForecastOutput, error) {
+	units := in.Units
+	if units == "" {
+		units = "celsius"
+	}
+	days := in.Days
+	if days == 0 {
+		days = 3
+	}
+	forecast := make([]ForecastDay, days)
+	conditions := []string{"sunny", "partly cloudy", "cloudy", "light rain", "clear", "overcast", "scattered showers"}
+	for i := range forecast {
+		high := 24.0 + float64(i)
+		low := 14.0 + float64(i)
+		if units == "fahrenheit" {
+			high = 75.0 + float64(i)*2
+			low = 57.0 + float64(i)*2
+		}
+		forecast[i] = ForecastDay{
+			Date:      "2025-01-0" + string(rune('1'+i)),
+			High:      high,
+			Low:       low,
+			Condition: conditions[i%len(conditions)],
+		}
+	}
+	return nil, GetForecastOutput{
+		Location: in.Location,
+		Units:    units,
+		Forecast: forecast,
+	}, nil
+}