Use SystemPath for PacketFilter

[chrisgeo]

Summary

Converts PacketFilter from URL to SystemPackage.FilePath for filesystem path handling, mirroring the pattern landed in apple#1480 for HostDNSResolver.

Changes

• configURL / anchorsURL → configPath / anchorsPath (FilePath)
• defaultConfigPath / defaultAnchorsPath now FilePath
• FileManager calls switch from atPath: url.path to atPath: path.string
• Test uses String(contentsOfFile: path.string, …) in place of String(contentsOf: URL, …)

Out of scope

• Process.executableURL = URL(fileURLWithPath: "/sbin/pfctl") — Foundation.Process requires URL, kept as-is.

Context

• Upstream tracking: apple/container discussion Good first issues - switching from URL to FilePath. apple/container#1481 (URL → FilePath migration)
• Reference PR: Use SystemPath for HostDNSResolver. apple/container#1480 (HostDNSResolver — sibling type, same pattern)
• Internal: CHAOS-1449 (one of 19 sub-issues under CHAOS-1448 epic)

Test plan

• swift test --filter PacketFilterTest — both tests pass (testRedirectRuleUpdate, testPacketFilterReinitialize)
• swift test --filter ContainerAPIClientTests — all 171 tests in 11 suites pass (no neighbor regressions)
• swift-format lint --strict --configuration .swift-format-nolint — clean
• Full project builds (warnings-as-errors: on)

Next steps

After merge here, forward to apple/container as the upstream PR (filing the upstream [Request]: TECH DEBT - Convert PacketFilter to FilePath issue first, mirroring apple#1479).

[linear]

CHAOS-1449