Add MCP conformance tests with client and server test suites (#767)

Implement conformance testing modules with comprehensive client/server test scenarios,
sync conversion for client/servlet code, CI integration, and documentation updates.

---------

Signed-off-by: Dariusz Jędrzejczyk <dariusz.jedrzejczyk@broadcom.com>

conformance-tests/VALIDATION_RESULTS.md

@@ -0,0 +1,82 @@
+# MCP Java SDK Conformance Test Validation Results
+
+## Summary
+
+**Server Tests:** 37/40 passed (92.5%)  
+**Client Tests:** 3/4 scenarios passed (9/10 checks passed)
+
+## Server Test Results
+
+### Passing (37/40)
+
+- **Lifecycle & Utilities (4/4):** initialize, ping, logging-set-level, completion-complete
+- **Tools (11/11):** All scenarios including progress notifications ✨
+- **Elicitation (10/10):** SEP-1034 defaults (5 checks), SEP-1330 enums (5 checks)
+- **Resources (4/6):** list, read-text, read-binary, templates-read
+- **Prompts (4/4):** list, simple, with-args, embedded-resource, with-image
+- **SSE Transport (2/2):** Multiple streams
+- **Security (1/2):** Localhost validation passes
+
+### Failing (3/40)
+
+1. **resources-subscribe** - Not implemented in SDK
+2. **resources-unsubscribe** - Not implemented in SDK  
+3. **dns-rebinding-protection** - Missing Host/Origin validation (1/2 checks)
+
+## Client Test Results
+
+### Passing (3/4 scenarios, 9/10 checks)
+
+- **initialize (1/1):** Protocol negotiation, clientInfo, capabilities
+- **tools_call (1/1):** Tool discovery and invocation
+- **elicitation-sep1034-client-defaults (5/5):** Default values for string, integer, number, enum, boolean
+
+### Partially Passing (1/4 scenarios, 1/2 checks)
+
+- **sse-retry (1/2 + 1 warning):** 
+  - ✅ Reconnects after stream closure
+  - ❌ Does not respect retry timing
+  - ⚠️ Does not send Last-Event-ID header (SHOULD requirement)
+
+**Issue:** Client treats `retry:` SSE field as invalid instead of parsing it for reconnection timing.
+
+## Known Limitations
+
+1. **Resource Subscriptions:** SDK doesn't implement `resources/subscribe` and `resources/unsubscribe` handlers
+2. **Client SSE Retry:** Client doesn't parse or respect the `retry:` field, reconnects immediately, and doesn't send Last-Event-ID header
+3. **DNS Rebinding Protection:** Missing Host/Origin header validation in server transport
+
+## Running Tests
+
+### Server
+```bash
+# Start server
+cd conformance-tests/server-servlet
+../../mvnw compile exec:java -Dexec.mainClass="io.modelcontextprotocol.conformance.server.ConformanceServlet"
+
+# Run tests (in another terminal)
+npx @modelcontextprotocol/conformance server --url http://localhost:8080/mcp --suite active
+```
+
+### Client
+```bash
+# Build
+cd conformance-tests/client-jdk-http-client
+../../mvnw clean package -DskipTests
+
+# Run all scenarios
+for scenario in initialize tools_call elicitation-sep1034-client-defaults sse-retry; do
+  npx @modelcontextprotocol/conformance client \
+    --command "java -jar target/client-jdk-http-client-0.18.0-SNAPSHOT.jar" \
+    --scenario $scenario
+done
+```
+
+## Recommendations
+
+### High Priority
+1. Fix client SSE retry field handling in `HttpClientStreamableHttpTransport`
+2. Implement resource subscription handlers in `McpStatelessAsyncServer`
+
+### Medium Priority
+3. Add Host/Origin validation in `HttpServletStreamableServerTransportProvider` for DNS rebinding protection