fix: mask ConversationMeta.last_preview on deserialization to prevent plaintext exposure

[Copilot]

from_json for ConversationMeta was unconditionally replacing any non-empty last_preview with the literal "(encrypted message)", causing StateDedupTest.ConversationMetaRoundTrip to fail. Based on review feedback, the correct fix is to preserve the masking behavior for privacy — preventing any sensitive plaintext previews from being resurfaced out of persisted state — but using a consistent masked value.

Change

Updated from_json for ConversationMeta to always set last_preview to "[encrypted message]" regardless of the stored value, and updated StateDedupTest.ConversationMetaRoundTrip to expect this masked value after a round-trip:

// Before
if (j.contains("last_preview")) {
    const auto preview = j.value("last_preview", "");
    v.last_preview = preview.empty() ? "" : "(encrypted message)";
} else {
    v.last_preview = "";
}

// After
// Never deserialize historical last_preview plaintext from disk into runtime state.
// This prevents resurfacing any sensitive previews that may have been persisted.
v.last_preview = "[encrypted message]";

---

💬 We'd love your input! Share your thoughts on Copilot coding agent in our 2 minute survey.

[Copilot]

Pull request overview

Fixes ConversationMeta JSON deserialization so last_preview survives a serialization round-trip (unblocking StateDedupTest.ConversationMetaRoundTrip and keeping state I/O symmetric with to_json).

Changes:

• Remove from_json(ConversationMeta&) logic that replaced any non-empty last_preview with "(encrypted message)".
• Deserialize last_preview directly via j.value("last_preview", "").

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[WilleLX1]

@copilot apply changes based on the comments in this thread

[Copilot]

@copilot apply changes based on the comments in this thread

Applied in commit 24f7467. from_json now always sets last_preview = "[encrypted message]" (with the comments from the reviewer's suggestion), and StateDedupTest.ConversationMetaRoundTrip now expects "[encrypted message]" after the round-trip.