Restrict API types visibility — types module is now private

Only Message, Role, MessageContent, ContentPart, ToolCall,
FunctionCall, Usage, ImageUrl are pub-exported from agent::api.

Internal types (ChatRequest, ChatCompletionChunk, ChunkChoice,
Delta, ReasoningConfig, ToolCallDelta, FunctionCallDelta) are
pub(crate) — invisible outside the crate.

All callers updated to import from agent::api:: instead of
agent::api::types::.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/tools/mod.rs

@@ -58,7 +58,7 @@ impl Tool {
 }
 
 // Re-export API wire types used by the agent turn loop
-use super::api::types::ToolCall;
+use super::api::ToolCall;
 
 /// A tool call in flight — metadata for TUI + JoinHandle for
 /// result collection and cancellation.