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

[syf2211]

Summary

Guard req.Session before calling InitializeParams() and Elicit() in the mcp-add tool handler so the gateway returns structured errors instead of panicking when session info is unavailable.

Fixes #442

Motivation

Some MCP transports/clients invoke mcp-add without populating req.Session. The handler dereferenced req.Session directly in the missing-config/secrets path and in remote OAuth handling, causing a nil pointer panic instead of a useful response.

Changes

• Add nil checks before req.Session.InitializeParams() when determining client name for missing secrets/config handling
• Allow remote OAuth handling to proceed when session is nil (delegate to getRemoteOAuthServerStatus, which already has safe fallbacks)
• Guard elicitation flow in getRemoteOAuthServerStatus when session is nil
• Add regression tests for nil-session paths

Tests

• go test ./pkg/gateway/ -run 'TestAddServerHandlerNilSession|TestGetRemoteOAuthServerStatusNilSession' -v -count=1 — PASS
• go test ./pkg/gateway/ -short -count=1 — PASS
• go vet ./pkg/gateway/... — PASS
• make lint — skipped (requires Docker in this environment)

Notes

• Pre-PR review: composer-2.5 reviewed; OAuth path updated per feedback to still invoke getRemoteOAuthServerStatus when session is nil.