Fix: Render MCP image outputs regardless of ordering

[Kbediako]

What?

• Render an MCP image output cell whenever any image block exists in CallToolResult.content.

Why?

• Tool results that include caption text before the image currently drop the image output cell.

How?

• Scan content for an ImageContent entry instead of only checking the first block.
• Add a unit test that covers text-before-image ordering.

Before

let image_output_cell = match call_tool_result.content.first() {
    Some(CallToolResultContent::Image(image_content)) => Some(history_cell::ImageOutputCell {
        context: Some(format!("Called {tool_name}")),
        caption: image_content.annotation().map(str::to_owned),
        data: image_content.data().to_owned(),
        mime_type: image_content.mime_type().to_owned(),
    }),
    _ => None,
};

After

let image_output_cell = call_tool_result
    .content
    .iter()
    .find_map(|content| match content {
        CallToolResultContent::Image(image_content) => Some(history_cell::ImageOutputCell {
            context: Some(format!("Called {tool_name}")),
            caption: image_content.annotation().map(str::to_owned),
            data: image_content.data().to_owned(),
            mime_type: image_content.mime_type().to_owned(),
        }),
        _ => None,
    });

Risk / Impact

• Low: only affects image cell creation for MCP tool results; no change for non-image outputs.

Tests

• just fmt
• cargo test -p codex-tui

Fixes #9814