fix(vsce): align model selection with code package and remove env logic from /config

[lewis617]

Summary

• Remove fast model dropdown from /model dialog to match code package behavior (code package only selects a single model)
• Align setModel flow with code package: use agent.setModel() for hot update + persist to ~/.wave/settings.json, instead of VS Code globalState save + agent destroy/recreate
• Remove all env var logic from /config dialog: no more env* fields in ConfigurationData, no env placeholder text, no maskSecret helper

Changes

Model selection alignment (/model)

• ModelDialog.tsx: removed fast model <select> dropdown, only model select remains
• messageHandler.ts: handleSetModel now calls updateAllSessionsModel(model) → agent.setModel() on all sessions
• chatSession.ts: added setModel() method delegating to agent.setModel()
• chatProvider.ts: added updateAllSessionsModel callback
• messageHandler.ts: handleGetConfiguredModels no longer sends currentFastModel

Env logic removal (/config)

• configurationService.ts: removed env* fields from interface, removed maskSecret(), removed env var reads from loadConfiguration()
• types/index.ts: removed env* fields from ConfigurationData type
• ConfigDialog.tsx: placeholders changed to plain text (no env references)
• StatusDialog.tsx: displays configurationData.* directly, no env fallback
• LoginDialog.tsx: serverUrl no longer falls back to envServerUrl, placeholder changed to plain text

Tests

• modelDropdown.test.tsx: removed all fast model assertions, added assertion that #fast-model-select does not exist
• model-dialog.demo.ts: removed env mock data and fast-model-select assertion

Test Results

• 191/191 VSCE unit tests pass
• 30/30 e2e demo tests pass
• 46/46 agent-sdk tests pass