fix(home-mixer): preserve ancestor and parent context for reply notifications (#205)

[Adityakk9031]

Summary

Fixes #205

Resolves an issue where opening a reply notification (or timeline requests with push-to-home focal replies) marshals the reply in isolation as an orphaned post, completely omitting the parent post being responded to.

Root Cause Analysis

1. Push-To-Home Marshalling Omission (push_to_home_marshaller.rs):
   • PushToHomePost holds both conversation_id (root) and in_reply_to_tweet_id (direct parent).
   • In push_to_home_marshaller, in_reply_to_tweet_id was checked solely to set is_root, but was never added to module_items.
   • Only root (from conversation_id) was added. If conversation_id == 0 or equaled the reply ID, root was None and module_items only contained the isolated focal reply.
   • For multi-hop replies in a conversation thread (root -> parent -> reply), only root was rendered and the immediate parent being responded to was completely omitted from the rendered items.
2. Conversation Marshaller Missing Parent Fallback (convo_marshaller.rs):
   • In convo_marshaller, if a post's ancestors array was empty, parent_tweets was empty, resulting in item_ids containing only [focal_id].
3. Timeline URT Routing Flaw (urt/mod.rs & reverse_chron_following/mod.rs):
   • The condition post.ancestors.is_empty() routed replies to post_marshaller::marshal_post (a standalone post) without checking post.in_reply_to_tweet_id == 0.
4. Hydration Omission (core_data_candidate_hydrator.rs):
   • When core data hydrated in_reply_to_tweet_id, candidate.ancestors was not populated, leaving replies vulnerable to orphan marshalling and premature filter dropping in OONRetweetReplyFilter.

Changes Made

• home-mixer/util/urt/push_to_home_marshaller.rs:
  • Added resolution for parent (post.in_reply_to_tweet_id) in marshal_push_to_home.
  • Assembles module_items in hierarchical order: [root_id, parent_id, focal_id] (deduping if root == parent), ensuring the parent post is always rendered above the reply.
  • Added unit tests covering root posts, single-hop replies, multi-hop replies, and missing conversation_id.
• home-mixer/util/urt/convo_marshaller.rs:
  • Added fallback to post.in_reply_to_tweet_id when post.ancestors is empty so conversation modules always render parent context.
  • Added unit tests for conversation module assembly and fallback behavior.
• home-mixer/util/urt/mod.rs & reverse_chron_following/mod.rs:
  • Enforced post.in_reply_to_tweet_id == 0 before routing to post_marshaller::marshal_post.
• home-mixer/candidate_hydrators/core_data_candidate_hydrator.rs:
  • Populated candidate.ancestors and ancestor_users when in_reply_to_tweet_id is present on core_data.