[Feature] Add unit tests for NetworkInspectorServer internal logic

[ConnorQi01]

Summary

NetworkInspectorServer still has no direct unit test coverage on origin/master. Its internal logic - certificate exchange medium resolution, device connection mapping, async client initialization, and start/stop error propagation - is currently exercised only through integration paths or adjacent command/manager tests.

This issue supersedes #2702.

Current coverage already in place

• Add Network Inspector manager lifecycle tests #2637 added test/extension/networkInspector/networkInspectorManager.test.ts for manager start/stop lifecycle behavior.
• Add Network Inspector command tests #2664 added test/extension/networkInspector/startStopCommands.test.ts for StartNetworkInspector and StopNetworkInspector command wrappers.

Those cover the command and manager portions originally mentioned here. The remaining gap is server-level coverage for src/extension/networkInspector/networkInspectorServer.ts.

Suggested scope

• Add test/extension/networkInspector/networkInspectorServer.test.ts using sinon + proxyquire, matching existing test style.
• Cover transformCertificateExchangeMediumToType for medium values 1, 2, and unknown/undefined fallback.
• Cover start() failure propagation when secure server config or server startup fails.
• Cover addConnection / removeConnection map management, including the async client.init() lifecycle where the client is returned before initialization completes and then inserted into connections.
• Cover disconnection during initialization so the lifecycle behavior is explicit and regression-tested.

Evidence

• origin/master contains test/extension/networkInspector/networkInspectorManager.test.ts and test/extension/networkInspector/startStopCommands.test.ts.
• origin/master does not contain test/extension/networkInspector/networkInspectorServer.test.ts.
• src/extension/networkInspector/networkInspectorServer.ts still contains untested branches around transformCertificateExchangeMediumToType, initialisePromise, addConnection, and removeConnection.

Validation

• npm test should include the new server test file and pass.
• Server tests should run without a live device or real network inspector connection.

Closes #2702