mcp: preserve structured content number precision (#1239)

CallToolResult currently decodes structuredContent through an any field,
which converts JSON numbers to float64 and silently rounds integers
outside the IEEE-754 safe range.

This change decodes structuredContent separately with UseNumber,
preserving wire numbers as json.Number while retaining the existing
decoded object, array, and primitive shapes. The field documentation now
describes that representation. Users who depend on the previous float64
behavior can temporarily restore it with
MCPGODEBUG=structuredcontentfloat64=1.

An end-to-end in-memory client/server regression covers 9007199254740993
from a low-level Server.AddTool result. Additional coverage verifies the
compatibility option restores the legacy rounded float64 value.

Verification:
- gofmt -l .
- go test ./...
- go vet ./...

Fixes #1236

---------

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

internal/docs/mcpgodebug.src.md

@@ -17,6 +17,16 @@
 
 ## `MCPGODEBUG` history
 
+### 1.9.0
+
+Options listed below were added and will be removed in the 1.11.0 version of the SDK.
+
+- `structuredcontentfloat64` added. If set to `1`, numbers in
+  `CallToolResult.StructuredContent` are decoded as `float64`, restoring the
+  previous behavior. The default behavior decodes numbers as `json.Number` so
+  that large integers and other exact JSON values do not silently lose
+  precision.
+
 ### 1.8.0
 
 Options listed below were added and will be removed in the 1.9.0 version of the SDK.

internal/json/json.go

@@ -44,6 +44,15 @@ func Unmarshal(data []byte, v any) error {
 	return NewDecoder(bytes.NewReader(data)).Decode(v)
 }
 
+func UnmarshalUseNumber(data []byte, v any) error {
+	if err := checkMaxDepth(data, defaultMaxDepth); err != nil {
+		return err
+	}
+	dec := NewDecoder(bytes.NewReader(data))
+	dec.dec.UseNumber()
+	return dec.Decode(v)
+}
+
 // checkMaxDepth scans data once and reports [errMaxDepthExceeded] if the
 // nesting of JSON objects and arrays exceeds maxDepth. It is a lightweight pass which
 // tracks '{' and '[' against '}' and ']' while skipping over the contents of strings.