fix(gateway): guard nil Session in mcp-add handler

[syf2211]

Summary

Guard all req.Session accesses in the mcp-add tool handler so a nil session returns a structured error instead of panicking.

Motivation

Fixes #442. Some transports/clients invoke gateway tools without populating req.Session. The handler called req.Session.InitializeParams() unguarded when reporting missing secrets/config or during OAuth elicitation, causing a nil pointer dereference.

Changes

• Add req.Session != nil guard before reading initialize params when missing secrets/config are reported
• Skip OAuth elicitation path when session is nil (remote server add flow)
• Wrap elicitation block in getRemoteOAuthServerStatus with a nil session check
• Add regression test TestAddServerHandler_NilSessionMissingConfig

Tests

go test ./pkg/gateway/ -run TestAddServerHandler_NilSessionMissingConfig -v
# PASS

Notes

• auditClientInfoFromSession already handles nil sessions
• Pre-PR review (composer-2.5): APPROVE

Fixes #442