Preserve content blocks and thinking origin in C store append

Round-trip persistent message content through the C session store bridge
instead of dropping it, including tool-use, tool-result, system, and
thinking blocks. Reconstruct borrowed C content blocks with arena-backed
helpers, carry message identity fields like API style/model/reasoning, and
add tests covering append marshalling and thinking signature origin
round-tripping.

1 files changed, 18 insertions, 0 deletions

diff --git a/libpanto/src/conversation.zig b/libpanto/src/conversation.zig
index 760f3ee..4338563 100644
--- a/libpanto/src/conversation.zig
+++ b/libpanto/src/conversation.zig
@@ -370,6 +370,24 @@ pub const Conversation = struct {
         });
     }
 
+    /// Append a message of any role from a slice of content blocks, with
+    /// optional usage. The general form behind `addUserMessage` /
+    /// `addAssistantMessage`; used to rebuild a persisted message (tool calls,
+    /// thinking, mixed blocks) losslessly. Ownership of the blocks transfers
+    /// to the conversation; the caller must not deinit them after this call.
+    pub fn addMessage(self: *Conversation, role: MessageRole, blocks: []const ContentBlock, usage: ?Usage) !void {
+        var content: std.ArrayList(ContentBlock) = .empty;
+        try content.ensureTotalCapacity(self.allocator, blocks.len);
+        for (blocks) |block| {
+            content.appendAssumeCapacity(block);
+        }
+        try self.messages.append(self.allocator, .{
+            .role = role,
+            .content = content,
+            .usage = usage,
+        });
+    }
+
     pub fn deinit(self: *Conversation) void {
         for (self.messages.items) |*msg| {
             msg.deinit(self.allocator);