feat: add Test connection tab to virtual server details drawer

[marekdano]

Summary

Adds a "Test connection" tab to the virtual server details drawer, running an MCP handshake against the virtual server's own endpoint and surfacing the result - negotiation path, server identity, advertised capabilities, credential source used, and component counts cross-checked against the virtual server's own aggregate (flagged on mismatch).

Closes IBM/mcp-context-forge#5650.

Blocked by IBM/mcp-context-forge#6370 — POST /v1/mcp-servers/test-handshake's SSRF allowlist only matches registered-gateway URLs (DbGateway.url), so it currently rejects a virtual server's own endpoint by design, regardless of the URL supplied. The frontend here is correct and calls the endpoint exactly as documented; it will start working the moment the backend accepts a virtual-server-scoped target. Opening as draft until #6370 lands — there's nothing left to fix on this side, but the feature can't be demoed/verified against a real virtual server until the backend gap is closed.

What's included

• src/api/servers.ts — serversApi.testHandshake() wrapper for POST /v1/mcp-servers/test-handshake.
• src/components/servers/HandshakeTestPanel.tsx — the handshake test panel: URL/headers form, result rendering (latency, negotiation path, server identity, capabilities, component-count badges with mismatch flagging against the virtual server's aggregate, credential source, per-failure-class actionable copy, size-capped raw preview).
• src/components/gateways/VirtualServerDetailsPanel.tsx — adds the Components / Test connection tab shell (following the #5648 drawer-tab pattern) and computes+forwards the virtual server's own aggregated tool/resource/prompt counts for mismatch comparison.
• Tests: HandshakeTestPanel.test.tsx (idle/success/failure/validation/cancel/unmount/capabilities/credential-source/failure-copy/count-mismatch, including a regression case built from a real backend response) and 5 new cases in VirtualServerDetailsPanel.test.tsx covering the tab wiring.

Test plan

• npx tsc -b — clean
• npm run lint / npm run format:check — clean
• npx vitest run — 184 files / 3080 tests passing
• Manual end-to-end verification — blocked on [Bug] Virtual server "Test connection" can never succeed - /v1/mcp-servers/test-handshake is scoped to registered gateways, not virtual servers IBM/mcp-context-forge#6370