fix: preserve unknown fields in policy.json during read-modify-write cycles

[rgarcia]

Summary

• Fix bug where DefaultGeolocationSetting (and any other non-struct fields) were silently dropped when AddExtension performed a read-modify-write cycle on policy.json
• Simplify Policy struct to only include fields we programmatically modify (ExtensionInstallForcelist, ExtensionSettings)
• Add custom UnmarshalJSON/MarshalJSON methods to preserve all unknown JSON fields

Context

This fixes an issue introduced by #136, which added DefaultGeolocationSetting to policy.json. The Go Policy struct didn't have this field, so when the JSON was unmarshaled and re-marshaled (e.g., during AddExtension), the field was silently dropped.

Solution

Instead of requiring every Chrome policy field to be defined in the Go struct, we now:

1. Only define struct fields for settings we need to modify programmatically
2. Preserve all other fields through an unknownFields map[string]json.RawMessage

This means you can add any Chrome policy setting to policy.json without Go code changes.

Test plan

• Added unit tests for field preservation through unmarshal/marshal cycles
• Tests verify unknown fields survive modification of known fields
• go build ./... passes
• go test ./lib/policy/... passes

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Note

Medium Risk
Touches JSON (un)marshaling for enterprise policy persistence; bugs here could corrupt or drop policy settings during writes, though changes are localized and covered by new tests.

Overview
Fixes policy.json read-modify-write behavior so non-modeled Chrome policy fields are preserved instead of being silently removed when updating extension policies.

This simplifies Policy to only the fields the code mutates (ExtensionInstallForcelist, ExtensionSettings) and adds custom MarshalJSON/UnmarshalJSON logic to round-trip all other keys via an unknownFields map. New unit tests cover unknown-field preservation, modifying known fields while retaining unknown ones, and ExtensionInstallForcelist round-tripping.

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