refactor: wave 2 high-impact consolidation

[datlechin]

Summary

Wave 2 of the refactoring plan from REFACTOR_TRACKER.md. Consolidates the highest-impact code duplication across 4 independent items, each touching different files with no overlap.

DUP-1: Extract SQLStatementScanner

• New SQLStatementScanner enum consolidates 3 duplicate SQL statement-boundary FSMs into one
• Deleted ~280 lines from MainContentCoordinator, MainContentCoordinator+MultiStatement, and SQLContextAnalyzer
• Bug fix: SQLContextAnalyzer's version was missing block comment (/* */), double-quote, and backtick handling
• 22 new tests covering all edge cases (strings, comments, escapes, cursor positioning)

DUP-2: Move AI provider resolution to AIProviderFactory

• Added resolveProvider(for:settings:) and resolveModel(for:config:settings:) to AIProviderFactory
• Deleted identical private methods from InlineSuggestionManager and AIChatViewModel (~60 lines)

DUP-3: Add DatabaseType.beginTransactionSQL

• New computed property replaces 5 inline switches + 1 helper method across 7 files
• Bug fix: MainContentCoordinator+TableOperations and MainContentCoordinator+SQLPreview gave MySQL/MariaDB "BEGIN" instead of "START TRANSACTION"

NOTIF-1: Centralize notification names

• Moved 12 scattered Notification.Name extensions from 6 files into new AppNotifications.swift
• Zero call-site changes needed — same extensions remain in scope

Test fixes

• Fixed flaky QueryHistoryStorage/HistoryDataProvider tests caused by clearAllHistory() cross-suite interference
• Added QueryHistoryStorage(isolatedForTesting:) and QueryHistoryManager(isolatedStorage:) for test isolation

Stats

• 27 files changed, ~480 lines removed, ~140 lines added (net -340)
• 3 new files, 24 modified files

Test plan

• All 22 new SQLStatementScannerTests pass
• All existing SQLContextAnalyzerTests pass (43 tests)
• Full test suite passes (2 consecutive runs, 0 failures)
• xcodebuild build succeeds
• Manual: multi-statement query with block comments in editor
• Manual: autocomplete inside block comment returns no completions

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