Persist text elements through TUI input and history (#9393)

Continuation of breaking up this PR
https://github.com/openai/codex/pull/9116

## Summary
- Thread user text element ranges through TUI/TUI2 input, submission,
queueing, and history so placeholders survive resume/edit flows.
- Preserve local image attachments alongside text elements and rehydrate
placeholders when restoring drafts.
- Keep model-facing content shapes clean by attaching UI metadata only
to user input/events (no API content changes).

## Key Changes
- TUI/TUI2 composer now captures text element ranges, trims them with
text edits, and restores them when submission is suppressed.
- User history cells render styled spans for text elements and keep
local image paths for future rehydration.
- Initial chat widget bootstraps accept empty `initial_text_elements` to
keep initialization uniform.
- Protocol/core helpers updated to tolerate the new InputText field
shape without changing payloads sent to the API.

codex-rs/app-server-test-client/src/main.rs

@@ -258,7 +258,7 @@ fn send_message_v2_with_policies(
         thread_id: thread_response.thread.id.clone(),
         input: vec![V2UserInput::Text {
             text: user_message,
-            // Plain text conversion has no UI element ranges.
+            // Test client sends plain text without UI element ranges.
             text_elements: Vec::new(),
         }],
         ..Default::default()
@@ -292,6 +292,7 @@ fn send_follow_up_v2(
         thread_id: thread_response.thread.id.clone(),
         input: vec![V2UserInput::Text {
             text: first_message,
+            // Test client sends plain text without UI element ranges.
             text_elements: Vec::new(),
         }],
         ..Default::default()
@@ -304,6 +305,7 @@ fn send_follow_up_v2(
         thread_id: thread_response.thread.id.clone(),
         input: vec![V2UserInput::Text {
             text: follow_up_message,
+            // Test client sends plain text without UI element ranges.
             text_elements: Vec::new(),
         }],
         ..Default::default()
@@ -477,6 +479,7 @@ impl CodexClient {
                 conversation_id: *conversation_id,
                 items: vec![InputItem::Text {
                     text: message.to_string(),
+                    // Test client sends plain text without UI element ranges.
                     text_elements: Vec::new(),
                 }],
             },