Use FilePath for content store client/server pair

[chrisgeo]

Summary

• Converts ContentStoreService (server) and RemoteContentStoreClient (client) from URL to SystemPackage.FilePath across the content-store XPC pair.
• Public API delta on the server actor:
  • ContentStoreService.init(root: URL, …) → init(root: FilePath, …)
  • get(digest:) async throws -> URL? → FilePath?
  • newIngestSession() async throws -> (id: String, ingestDir: URL) → (id: String, ingestDir: FilePath)
• RPC contract co-updated in lockstep: ContentServiceHarness now serializes paths via FilePath.string (replacing URL.path(percentEncoded: false)); the wire format on XPCMessageKey.contentPath and .directory is unchanged (still a plain absolute path string), so client and server stay compatible.
• Single construction site for ContentStoreService updated at Sources/Plugins/CoreImages/ImagesHelper.swift:116 — passes FilePath(root.path(percentEncoded: false)) since appRoot upstream is still URL.
• Client RemoteContentStoreClient: private _get(digest:) is now FilePath-typed and decodes XPC strings via FilePath(path) instead of URL(filePath: path). The public protocol surface for ContentStore (i.e. ingest(_ body: @Sendable @escaping (URL) async throws -> Void) and newIngestSession() -> (id, URL)) is pinned by the upstream ContainerizationOCI.ContentStore protocol and stays URL — see boundary notes below.
• No Codable wire-format change. ContentStoreService is an actor with non-serialized stored properties; XPC payloads remain plain path strings.

URL boundaries preserved (with inline comments justifying each leak):

• ContentStoreService.init → LocalContentStore(path: URL(filePath: self.root.string)) — the upstream ContainerizationOCI.LocalContentStore constructor only accepts URL.
• RemoteContentStoreClient.get(digest:) → LocalContent(path: URL(filePath: path.string)) — the upstream ContainerizationOCI.LocalContent constructor only accepts URL.
• ContentStoreService.get(digest:) and .newIngestSession() convert URLs returned by LocalContentStore back into FilePath via FilePath(url.path(percentEncoded: false)) at the Containerization library boundary.
• RemoteContentStoreClient.newIngestSession() and .ingest(_:) keep their URL signatures unchanged because they are required by the external ContentStore protocol; changing them would break protocol conformance. Same precedent as PR Use FilePath for SnapshotStore #25's directorySize / URLResourceKey leak.

Closes CHAOS-1455. Sibling of #26 (Bundle), part of CHAOS-1448 epic.

Type of Change

• Refactor / tech debt (URL → FilePath migration)

Testing

• `swift build` succeeds — clean compile, no warnings (post-rebase verified at SHA 74395be).
• `swift test --filter ContainerImagesService` — no failures (0 tests matched; module has no dedicated unit tests today, same as PR Use FilePath for SnapshotStore #25).
• lsp_diagnostics clean on all four touched files.
• Manual: `container image pull` exercises `ContentStoreService.newIngestSession` and `RemoteContentStoreClient.ingest` end-to-end; XPC-string round-trip unchanged because wire format is the same plain path string.
• Manual: `container image rm` / `container system prune` exercises `delete(digests:)` / `delete(keeping:)` paths (untouched by this PR).

[linear]

CHAOS-1455

CHAOS-1448