chore: use anyhow::Result for all app-server integration tests

[owenlin0]

There's a lot of visual noise in app-server's integration tests due to the number of .expect("<some_msg>") lines which are largely redundant / not very useful. Clean them up by using anyhow::Result + ? consistently.

Replaces the existing pattern of:

    let codex_home = TempDir::new().expect("create temp dir");
    create_config_toml(codex_home.path()).expect("write config.toml");

    let mut mcp = McpProcess::new(codex_home.path())
        .await
        .expect("spawn mcp process");
    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize())
        .await
        .expect("initialize timeout")
        .expect("initialize request");

With:

    let codex_home = TempDir::new()?;
    create_config_toml(codex_home.path())?;

    let mut mcp = McpProcess::new(codex_home.path()).await?;
    timeout(DEFAULT_READ_TIMEOUT, mcp.initialize()).await??;

[pakrym-oai]

can be removed?

[pakrym-oai]

or is it a parsing test?

[owenlin0]

yea looks like it checks that the response parses. we do it a few times in this file

[owenlin0]

I don't want to change the assertions in this pure refactor PR, so i'll leave this alone