[BUG] Add durable per-view state persistence for parallel tabs #984

Problem

Parallel mode webview instances lack durable per-view persistence. When a user:

1. Opens multiple parallel tabs with different configurations (mode, API config)
2. Reloads the webview or restarts VS Code
3. All tabs lose their individual state and revert to defaults

Previously, persistence relied on scattered dynamic __view_state_* keys that were not formally registered in the global settings schema. This meant:

• No guaranteed reload across VS Code restarts
• No bounded cleanup (keys could accumulate indefinitely)
• No proper hydration through ContextProxy.initialize()
• Full apiConfiguration (including secrets) was stored, risking secret leaks

Solution

Introduce a formal registered global setting key viewStates in globalSettingsSchema:

• Stores only non-secret selections: mode, currentApiConfigName, updatedAt
• Included in GLOBAL_STATE_KEYS for proper ContextProxy hydration on startup
• Bounded cleanup: keeps most recent 50 entries, prunes older ones
• Hydrated via loadViewState() which resolves profile settings through ProviderSettingsManager.getProfile()

Scope

• New registered global setting key viewStates in globalSettingsSchema
• Bounded persistence with timestamp-based pruning (max 50 entries)
• ContextProxy hydration on startup for all parallel instances
• Profile resolution via ProviderSettingsManager
• No full apiConfiguration or API keys stored — only non-secret selections

Tests

• Persistence tests: 24 passed
• Full core run: 129 files passed / 2184 passed / 9 skipped