Revert to v2.18.10 - Remove session persistence (v2.19.0-v2.19.5) (#322)

After 5 consecutive hotfix attempts, session persistence has proven
architecturally incompatible with the MCP SDK. Rolling back to last
known stable version.

## Removed
- 16 new files (session types, docs, tests, planning docs)
- 1,100+ lines of session persistence code
- Session restoration hooks and lifecycle events
- Retry policy and warm-start implementations

## Restored
- Stable v2.18.10 codebase
- Library export fields (from PR #310)
- All core MCP functionality

## Breaking Changes
- Session persistence APIs removed
- onSessionNotFound hook removed
- Session lifecycle events removed

This reverts commits fe13091 through 1d34ad8.
Restores commit 4566253 (v2.18.10, PR #310).

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

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

tests/unit/http-server-session-management.test.ts

@@ -631,16 +631,15 @@ describe('HTTP Server Session Management', () => {
   describe('Transport Management', () => {
     it('should handle transport cleanup on close', async () => {
       server = new SingleSessionHTTPServer();
-
-      // Test the transport cleanup mechanism by calling removeSession directly
+      
+      // Test the transport cleanup mechanism by setting up a transport with onclose
       const sessionId = 'test-session-id-1234-5678-9012-345678901234';
       const mockTransport = {
         close: vi.fn().mockResolvedValue(undefined),
         sessionId,
-        onclose: undefined as (() => void) | undefined,
-        onerror: undefined as ((error: Error) => void) | undefined
+        onclose: null as (() => void) | null
       };
-
+      
       (server as any).transports[sessionId] = mockTransport;
       (server as any).servers[sessionId] = {};
       (server as any).sessionMetadata[sessionId] = {
@@ -648,16 +647,18 @@ describe('HTTP Server Session Management', () => {
         createdAt: new Date()
       };
 
-      // Directly call removeSession to test cleanup behavior
-      await (server as any).removeSession(sessionId, 'transport_closed');
+      // Set up the onclose handler like the real implementation would
+      mockTransport.onclose = () => {
+        (server as any).removeSession(sessionId, 'transport_closed');
+      };
 
-      // Verify cleanup completed
+      // Simulate transport close
+      if (mockTransport.onclose) {
+        await mockTransport.onclose();
+      }
+
+      // Verify cleanup was triggered
       expect((server as any).transports[sessionId]).toBeUndefined();
-      expect((server as any).servers[sessionId]).toBeUndefined();
-      expect((server as any).sessionMetadata[sessionId]).toBeUndefined();
-      expect(mockTransport.close).toHaveBeenCalled();
-      expect(mockTransport.onclose).toBeUndefined();
-      expect(mockTransport.onerror).toBeUndefined();
     });
 
     it('should handle multiple concurrent sessions', async () => {