refactor(plugin-redis): replace recursive DisclosureGroup with OutlineGroup

[datlechin]

Summary

• RedisKeyTreeView previously rendered the namespace tree via a recursive renderNodes(_:) -> AnyView function with nested ForEach + DisclosureGroup. The AnyView wrap was needed because Swift can't express the recursive view-tree type, but it kills SwiftUI's view diffing.
• Replace with SwiftUI's native OutlineGroup(nodes, children: \.children) in 91 LoC. No AnyView, no recursion in code — SwiftUI handles arbitrary depth.
• Drop the explicit expandedPrefixes: Set<String> binding from the viewmodel and the call site. SwiftUI manages expansion state per branch identifier (via RedisKeyNode.id); state persists across re-renders for branches whose ID survives, which matches what the explicit set was approximating.
• Add var children: [RedisKeyNode]? computed property to RedisKeyNode (returns the children array for .namespace, nil for .key) so OutlineGroup can navigate.

Picked SwiftUI OutlineGroup over AppKit NSOutlineView because the surrounding sidebar is a SwiftUI List with Sections; embedding NSOutlineView would create a hybrid with its own scroll view, different selection chrome, and different keyboard nav.

Stacked on #1131. Review that one first.

Test plan

• Connect to Redis with hierarchical keys (user:1, user:2:profile, cache:foo, session:abc)
• Tree shows namespaces with folder icons + key counts
• Click chevron → namespace expands; click again → collapses
• Arrow keys (Right/Left when row is focused) expand/collapse natively
• Click namespace label (not chevron) → opens namespace browse view
• Click key row → opens key value editor
• Type in search → tree filters; expand state preserved for surviving namespaces
• Clear search → tree returns to full
• Empty database → "No keys" caption
• Database with >50,000 keys → "Showing first 50,000 keys" footer
• Switch Redis connection → tree resets cleanly
• Other sidebar sections (tables, views) scroll together with Redis tree as one unified list

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