Fix Sendable constraint error - add tryRemovePendingRequest helper

The removePendingRequest() method returns AnyPendingRequest? which is not
Sendable. This caused errors when calling from Task context in the timeout
wrapper.

Solution: Added tryRemovePendingRequest() that returns Bool instead, which
is Sendable and allows us to check if removal succeeded across actor boundaries.

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

Co-Authored-By: Claude <noreply@anthropic.com>

Author: dcherrera

Sources/MCP/Client/Client.swift

@@ -353,7 +353,7 @@ public actor Client {
                             // If send fails, try to remove the pending request.
                             // Resume with the send error only if we successfully removed the request,
                             // indicating the response handler hasn't processed it yet.
-                            if await self.removePendingRequest(id: request.id) != nil {
+                            if await self.tryRemovePendingRequest(id: request.id) {
                                 continuation.resume(throwing: error)
                             }
                             // Otherwise, the request was already removed by the response handler
@@ -378,7 +378,7 @@ public actor Client {
                 return result
             } catch {
                 // Clean up pending request on timeout
-                _ = await self.removePendingRequest(id: request.id)
+                await self.tryRemovePendingRequest(id: request.id)
                 throw error
             }
         }
@@ -396,6 +396,17 @@ public actor Client {
         return pendingRequests.removeValue(forKey: id)
     }
 
+    private func hasPendingRequest(id: ID) -> Bool {
+        return pendingRequests[id] != nil
+    }
+
+    @discardableResult
+    private func tryRemovePendingRequest(id: ID) -> Bool {
+        let existed = pendingRequests[id] != nil
+        pendingRequests.removeValue(forKey: id)
+        return existed
+    }
+
     // MARK: - Batching
 
     /// A batch of requests.