fix: add implementation description metadata (#981)

## Summary
- add the optional `description` field to `mcp.Implementation`
- keep the field omitted when empty
- cover JSON marshal/unmarshal behavior with a focused regression test

Fixes #977.

## To verify
- `go test ./mcp -run TestImplementationDescriptionJSON -count=1`
- `go test ./mcp -run TestServerRequest_PerRequestAccessors -count=1`
- `go test ./mcp -count=1`
- `go test ./...`
- `git diff --check`

Co-authored-by: Guglielmo Colombo <guglielmoc@google.com>

mcp/protocol.go

@@ -1935,16 +1935,18 @@ type ElicitationCompleteParams struct {
 func (x *ElicitationCompleteParams) isParams()   {}
 func (x *ElicitationCompleteParams) isNil() bool { return x == nil }
 
-// An Implementation describes the name and version of an MCP implementation, with an optional
-// title for UI representation.
+// An Implementation describes the name and version of an MCP implementation, with
+// optional display metadata.
 type Implementation struct {
 	// Intended for programmatic or logical use, but used as a display name in past
 	// specs or fallback (if title isn't present).
 	Name string `json:"name"`
 	// Intended for UI and end-user contexts — optimized to be human-readable and
 	// easily understood, even by those unfamiliar with domain-specific terminology.
-	Title   string `json:"title,omitempty"`
-	Version string `json:"version"`
+	Title string `json:"title,omitempty"`
+	// A human-readable description of the implementation.
+	Description string `json:"description,omitempty"`
+	Version     string `json:"version"`
 	// WebsiteURL for the server, if any.
 	WebsiteURL string `json:"websiteUrl,omitempty"`
 	// Icons for the Server, if any.