fix(ios): clean up Swift 6 build warnings and actor isolation in driver streaming

[datlechin]

Summary

Five small build issues that surfaced (warnings + one Swift 6 strict-concurrency error). All low risk, mechanical fixes.

ConnectionCoordinator: unused session binding

switchDatabase(to:) did guard let session, ... but never read session directly — the function uses appState.connectionManager.session(for:) to fetch a fresh handle later. The binding was a nil-check disguised as a let. Replaced with guard session != nil.

Three drivers: try on non-throwing call

MySQLDriver, PostgreSQLDriver, SQLiteDriver all call actor.fetchNextRow(options:columns:) from executeStreaming. The actor method signature returns [Cell]? without throws, so try await is a no-op. Swift 6 warns "No calls to throwing functions occur within 'try' expression". Drop the try.

StreamingExporter: actor-isolated fileExtension

ExportFormat.fileExtension extension is defined in StreamingExporter.swift at file scope. Under Swift 6 the file scope inherits the module's default isolation (MainActor in this build), so the extension property became MainActor-isolated. actor StreamingExporter then can't read it without await. Mark the property nonisolated since it's a pure pattern-match with no shared state.

Test plan

• Build: zero warnings/errors from the listed files
• Run a streaming export (CSV / JSON / SQL) end-to-end - file generated with correct extension
• Switch databases on a MySQL / PostgreSQL connection - works as before
• Run a SELECT that streams >100 rows on each driver - rows arrive in order