Fix: Render MCP image outputs regardless of ordering

[Kbediako]

What?

• Render an MCP image output cell whenever a decodable image block exists in CallToolResult.content (including text-before-image or malformed image before valid image).

Why?

• Tool results that include caption text before the image currently drop the image output cell.
• A malformed image block can also suppress later valid image output.

How?

• Iterate content and return the first successfully decoded image instead of only checking the first block.
• Add unit tests that cover text-before-image ordering and invalid-image-before-valid.

Before

let image = match result {
    Ok(mcp_types::CallToolResult { content, .. }) => {
        if let Some(mcp_types::ContentBlock::ImageContent(image)) = content.first() {
            // decode image (fails -> None)
        } else {
            None
        }
    }
    _ => None,
}?;

After

let image = result
    .as_ref()
    .ok()?
    .content
    .iter()
    .find_map(decode_mcp_image)?;

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
• Rerun after branch update (2026-01-27): just fmt, cargo test -p codex-tui

Fixes #9814

[joshka-oai]

Added a small set of extra MCP helpers to make it possible to manually test this in b9e3749

[joshka-oai]

Manual testing: MCP image tool result rendering (Codex TUI)

Build the rmcp stdio test server binary:

cd codex-rs
cargo build -p codex-rmcp-client --bin test_stdio_server

Register the server as an MCP server (absolute path to the built binary):

codex mcp add mcpimg -- /Users/joshka/code/codex-pr-review/codex-rs/target/debug/test_stdio_server

Then in Codex TUI, ask it to call:

• mcpimg.image_scenario({"scenario":"image_only"})
• mcpimg.image_scenario({"scenario":"text_then_image","caption":"Here is the image:"})
• mcpimg.image_scenario({"scenario":"invalid_base64_then_image"})
• mcpimg.image_scenario({"scenario":"invalid_image_bytes_then_image"})
• mcpimg.image_scenario({"scenario":"multiple_valid_images"})
• mcpimg.image_scenario({"scenario":"image_then_text","caption":"Here is the image:"})
• mcpimg.image_scenario({"scenario":"text_only","caption":"Here is the image:"})

Expected:

• You should see an extra history cell: "tool result (image output)" when the
  tool result contains at least one decodable image block (even if earlier
  blocks are text or invalid images).