[chore] Update interactionPolicy sub-policy parsing in line with documented defaults (#4229)

# Description

> If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.
>
> If this is a documentation change, please briefly describe what you've changed and why.

Brings our parsing of unset sub-policies in line with the defaults documented here: https://docs.gotosocial.org/en/v0.19.1/federation/interaction_policy/#defaults-per-sub-policy

Closes https://codeberg.org/superseriousbusiness/gotosocial/issues/4146

Part of https://codeberg.org/superseriousbusiness/gotosocial/issues/4026

## Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: `[ ]` -> `[x]`

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

- [x] I/we have read the [GoToSocial contribution guidelines](https://codeberg.org/superseriousbusiness/gotosocial/src/branch/main/CONTRIBUTING.md).
- [x] I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
- [x] I/we have not leveraged AI to create the proposed changes.
- [x] I/we have performed a self-review of added code.
- [x] I/we have written code that is legible and maintainable by others.
- [x] I/we have commented the added code, particularly in hard-to-understand areas.
- [ ] I/we have made any necessary changes to documentation.
- [x] I/we have added tests that cover new code.
- [x] I/we have run tests and they pass locally with the changes.
- [x] I/we have run `go fmt ./...` and `golangci-lint run`.

Reviewed-on: https://codeberg.org/superseriousbusiness/gotosocial/pulls/4229
Co-authored-by: tobi <tobi.smethurst@protonmail.com>
Co-committed-by: tobi <tobi.smethurst@protonmail.com>

internal/ap/extract.go

@@ -1113,6 +1113,9 @@ func ExtractVisibility(addressable Addressable, actorFollowersURI string) (gtsmo
 // Will be nil (default policy) for Statusables that have no policy
 // set on them, or have a null policy. In such a case, the caller
 // should assume the default policy for the status's visibility level.
+//
+// Sub-policies of the returned policy, eg., CanLike, CanReply, may
+// each be nil if they were not set on the interaction policy.
 func ExtractInteractionPolicy(
 	statusable Statusable,
 	owner *gtsmodel.Account,
@@ -1139,6 +1142,8 @@ func ExtractInteractionPolicy(
 		return nil
 	}
 
+	// There's a policy key/value
+	// set, extract sub-policies.
 	return &gtsmodel.InteractionPolicy{
 		CanLike:     extractCanLike(policy.GetGoToSocialCanLike(), owner),
 		CanReply:    extractCanReply(policy.GetGoToSocialCanReply(), owner),
@@ -1146,67 +1151,82 @@ func ExtractInteractionPolicy(
 	}
 }
 
+// Returns either a parsed CanLike sub-policy, or nil
+// if canLike is not set, ie., if this post is from an
+// instance that doesn't know / care about canLike.
 func extractCanLike(
 	prop vocab.GoToSocialCanLikeProperty,
 	owner *gtsmodel.Account,
-) gtsmodel.PolicyRules {
+) *gtsmodel.PolicyRules {
 	if prop == nil || prop.Len() != 1 {
-		return gtsmodel.PolicyRules{}
+		return nil
 	}
 
 	propIter := prop.At(0)
 	if !propIter.IsGoToSocialCanLike() {
-		return gtsmodel.PolicyRules{}
-	}
-
-	return extractPolicyRules(propIter.Get(), owner)
-}
-
-func extractCanReply(
-	prop vocab.GoToSocialCanReplyProperty,
-	owner *gtsmodel.Account,
-) gtsmodel.PolicyRules {
-	if prop == nil || prop.Len() != 1 {
-		return gtsmodel.PolicyRules{}
-	}
-
-	propIter := prop.At(0)
-	if !propIter.IsGoToSocialCanReply() {
-		return gtsmodel.PolicyRules{}
-	}
-
-	return extractPolicyRules(propIter.Get(), owner)
-}
-
-func extractCanAnnounce(
-	prop vocab.GoToSocialCanAnnounceProperty,
-	owner *gtsmodel.Account,
-) gtsmodel.PolicyRules {
-	if prop == nil || prop.Len() != 1 {
-		return gtsmodel.PolicyRules{}
-	}
-
-	propIter := prop.At(0)
-	if !propIter.IsGoToSocialCanAnnounce() {
-		return gtsmodel.PolicyRules{}
+		return nil
 	}
 
 	withRules := propIter.Get()
 	if withRules == nil {
-		return gtsmodel.PolicyRules{}
+		return nil
 	}
 
-	return extractPolicyRules(propIter.Get(), owner)
+	return extractPolicyRules(withRules, owner)
+}
+
+// Returns either a parsed CanReply sub-policy, or nil
+// if canReply is not set, ie., if this post is from an
+// instance that doesn't know / care about canReply.
+func extractCanReply(
+	prop vocab.GoToSocialCanReplyProperty,
+	owner *gtsmodel.Account,
+) *gtsmodel.PolicyRules {
+	if prop == nil || prop.Len() != 1 {
+		return nil
+	}
+
+	propIter := prop.At(0)
+	if !propIter.IsGoToSocialCanReply() {
+		return nil
+	}
+
+	withRules := propIter.Get()
+	if withRules == nil {
+		return nil
+	}
+
+	return extractPolicyRules(withRules, owner)
+}
+
+// Returns either a parsed CanAnnounce sub-policy, or nil
+// if canAnnounce is not set, ie., if this post is from an
+// instance that doesn't know / care about canAnnounce.
+func extractCanAnnounce(
+	prop vocab.GoToSocialCanAnnounceProperty,
+	owner *gtsmodel.Account,
+) *gtsmodel.PolicyRules {
+	if prop == nil || prop.Len() != 1 {
+		return nil
+	}
+
+	propIter := prop.At(0)
+	if !propIter.IsGoToSocialCanAnnounce() {
+		return nil
+	}
+
+	withRules := propIter.Get()
+	if withRules == nil {
+		return nil
+	}
+
+	return extractPolicyRules(withRules, owner)
 }
 
 func extractPolicyRules(
 	withRules WithPolicyRules,
 	owner *gtsmodel.Account,
-) gtsmodel.PolicyRules {
-	if withRules == nil {
-		return gtsmodel.PolicyRules{}
-	}
-
+) *gtsmodel.PolicyRules {
 	// Check for `automaticApproval` and
 	// `manualApproval` properties first.
 	var (
@@ -1216,7 +1236,7 @@ func extractPolicyRules(
 	if (automaticApproval != nil && automaticApproval.Len() != 0) ||
 		(manualApproval != nil && manualApproval.Len() != 0) {
 		// At least one is set, use these props.
-		return gtsmodel.PolicyRules{
+		return &gtsmodel.PolicyRules{
 			AutomaticApproval: extractPolicyValues(automaticApproval, owner),
 			ManualApproval:    extractPolicyValues(manualApproval, owner),
 		}
@@ -1226,7 +1246,7 @@ func extractPolicyRules(
 	// and `withApproval` properties.
 	//
 	// TODO: Remove this in GtS v0.21.0.
-	return gtsmodel.PolicyRules{
+	return &gtsmodel.PolicyRules{
 		AutomaticApproval: extractPolicyValues(withRules.GetGoToSocialAlways(), owner),
 		ManualApproval:    extractPolicyValues(withRules.GetGoToSocialApprovalRequired(), owner),
 	}