[Client] add client conformance test suite

[chr-hertel]

Summary

• Adds client conformance testing using @modelcontextprotocol/conformance client mode
• New tests/Conformance/client.php client executable handling initialize, tools_call, and elicitation scenarios
• New client-conformance-tests Makefile target and client-conformance CI job
• Baselines auth (22), SSE-retry, and elicitation scenarios as expected failures (missing OAuth and server-to-client SSE request support)
• Fixes empty array/object JSON serialization in ClientCapabilities and CallToolRequest ([] → {})

Test plan

• make client-conformance-tests passes (baseline check green)
• make unit-tests passes (600 tests)
• make phpstan passes
• make cs passes

🤖 Generated with Claude Code