Use SystemPath for PluginConfig

[chrisgeo]

Summary

Converts PluginConfig.init from URL to SystemPackage.FilePath, following the established migration pattern (apple#1480, apple#1518).

Changes

• PluginConfig.init?(configURL: URL) throws → init?(configPath: FilePath) throws
• DefaultPluginFactory.findConfigURL → findConfigPath (return type URL? → FilePath?)
• Both internal call sites updated (DefaultPluginFactory, AppBundlePluginFactory)
• configPath.extension (FilePath, String?) replaces configURL.pathExtension

Optional handling

FilePath.extension returns String? while URL.pathExtension returns String. The switch configPath.extension arms (case "toml", case "json") only match the non-nil cases; nil falls through to default: return nil. Safe by construction — and the nil case is unreachable in practice since findConfigPath only returns paths for config.toml or config.json.

Out of scope (deliberately)

• findConfigPath still takes directory: URL (not FilePath). The directory comes from FileManager.contentsOfDirectory which produces URL, and broader PluginFactory conversion is tracked separately as CHAOS-1464. The bridge from URL → FilePath happens at the return point only.
• Plugin.binaryURL: URL and Plugin.resourceURL: URL? unchanged — they're passed directly to Process.executableURL (Foundation requires URL).

Wire format

PluginConfig is Codable but is only ever decoded from on-disk TOML/JSON config files (read-only). Plugin (which embeds PluginConfig) is encoded for XPC, but Plugin's stored properties are unchanged by this PR. No wire-format break.

Context

• Upstream tracking: apple/container discussion Good first issues - switching from URL to FilePath. apple/container#1481
• Reference PRs: Use SystemPath for HostDNSResolver. apple/container#1480, Use SystemPath for PacketFilter. apple/container#1518
• Internal: CHAOS-1458

Test plan

• swift test --filter PluginConfigTest — 4/4 tests pass
• swift test --filter ContainerPluginTests — 28/28 tests pass (5 suites)
• swift-format strict lint clean

[linear]

CHAOS-1458