Fix spec types test compatibility after upstream merge

Added FixSpecServerCapabilities and FixSpecInitializeResult type helpers
to handle index signature requirements for ServerCapabilities that differ
between the spec types (no index signature) and SDK types (passthrough
with index signature).

Author: LucaButBoring

src/spec.types.test.ts

@@ -67,6 +67,11 @@ type FixSpecClientCapabilities<T> = T extends { elicitation?: object }
     ? Omit<T, 'elicitation'> & { elicitation?: Record<string, unknown> }
     : T;
 
+// Targeted fix: in spec, ServerCapabilities needs index signature to match SDK's passthrough
+type FixSpecServerCapabilities<T> = T & { [x: string]: unknown };
+
+type FixSpecInitializeResult<T> = T extends { capabilities: infer C } ? T & { capabilities: FixSpecServerCapabilities<C> } : T;
+
 type FixSpecInitializeRequestParams<T> = T extends { capabilities: infer C }
     ? Omit<T, 'capabilities'> & { capabilities: FixSpecClientCapabilities<C> }
     : T;
@@ -531,15 +536,15 @@ const sdkTypeChecks = {
         sdk = spec;
         spec = sdk;
     },
-    InitializeResult: (sdk: SDKTypes.InitializeResult, spec: SpecTypes.InitializeResult) => {
+    InitializeResult: (sdk: SDKTypes.InitializeResult, spec: FixSpecInitializeResult<SpecTypes.InitializeResult>) => {
         sdk = spec;
         spec = sdk;
     },
     ClientCapabilities: (sdk: SDKTypes.ClientCapabilities, spec: FixSpecClientCapabilities<SpecTypes.ClientCapabilities>) => {
         sdk = spec;
         spec = sdk;
     },
-    ServerCapabilities: (sdk: SDKTypes.ServerCapabilities, spec: SpecTypes.ServerCapabilities) => {
+    ServerCapabilities: (sdk: SDKTypes.ServerCapabilities, spec: FixSpecServerCapabilities<SpecTypes.ServerCapabilities>) => {
         sdk = spec;
         spec = sdk;
     },