fix: change JSON-RPC request ID type from u32 to i64 (#416)

The JSON-RPC 2.0 specification allows the ID field to be any JSON number,
including negative integers and large values. The previous u32 implementation
was limited to 0-4,294,967,295 and couldn't handle negative IDs.

Changes:
- Changed NumberOrString::Number from u32 to i64 to support full JSON number range
- Updated deserializer to handle both signed and unsigned integers
- Modified AtomicU32Provider to use AtomicU64 internally with i64 conversion
- Fixed progress token handling in meta.rs for i64 values
- Added comprehensive test for negative and large request IDs

This ensures full compliance with the JSON-RPC 2.0 specification.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-authored-by: Claude <noreply@anthropic.com>

crates/rmcp/src/model/meta.rs

@@ -116,9 +116,19 @@ impl Meta {
     pub fn get_progress_token(&self) -> Option<ProgressToken> {
         self.0.get(PROGRESS_TOKEN_FIELD).and_then(|v| match v {
             Value::String(s) => Some(ProgressToken(NumberOrString::String(s.to_string().into()))),
-            Value::Number(n) => n
-                .as_u64()
-                .map(|n| ProgressToken(NumberOrString::Number(n as u32))),
+            Value::Number(n) => {
+                if let Some(i) = n.as_i64() {
+                    Some(ProgressToken(NumberOrString::Number(i)))
+                } else if let Some(u) = n.as_u64() {
+                    if u <= i64::MAX as u64 {
+                        Some(ProgressToken(NumberOrString::Number(u as i64)))
+                    } else {
+                        None
+                    }
+                } else {
+                    None
+                }
+            }
             _ => None,
         })
     }