Support typed content in contextual prompt fragments (#54776)

## Why

Contextual fragments and extension-owned world-state sections currently return strings, limiting them to text content.

## What changed

- Return `ContentItem` from fragment bodies and rendering, preserving classification and attribution. Apply text markers only to `InputText`; pass other content through unchanged.
- Allow extension prompt and world-state contributions to carry typed content, and preserve media in context-window hints.
- Hash non-text world-state fragments by their serialized content and role while retaining the existing text hashing behavior.
- Add shared content-to-tool-output conversion that preserves image references, image detail, and audio.

## Testing

Add coverage for typed world-state hashing and preservation of image content and attribution. Expand code-mode output tests to cover input text, output text, file-backed and inline images, and audio.

GitOrigin-RevId: 2ed00d466a3f169b151202f3eef71e667ea64be1

codex-rs/protocol/tests/support/BUILD.bazel

@@ -0,0 +1,7 @@
+load("//:defs.bzl", "codex_rust_crate")
+
+codex_rust_crate(
+    name = "support",
+    crate_name = "codex_protocol_test_support",
+    crate_srcs = glob(["*.rs"]),
+)

codex-rs/protocol/tests/support/Cargo.toml

@@ -0,0 +1,16 @@
+[package]
+name = "codex-protocol-test-support"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lib]
+path = "lib.rs"
+test = false
+doctest = false
+
+[lints]
+workspace = true
+
+[dependencies]
+codex-protocol = { workspace = true }

codex-rs/protocol/tests/support/lib.rs

@@ -0,0 +1,21 @@
+//! Text assertions for protocol content, shared only by test targets.
+
+use codex_protocol::models::ContentItem;
+
+/// Assertion helpers for tests that require text rather than media content.
+/// Implementations should fail at the calling assertion for non-text content.
+pub trait ContentItemTestExt {
+    /// Extracts text, panicking at the call site when the content is not text.
+    #[track_caller]
+    fn expect_text(self) -> String;
+}
+
+impl ContentItemTestExt for ContentItem {
+    #[track_caller]
+    fn expect_text(self) -> String {
+        match self.into_text() {
+            Some(text) => text,
+            None => panic!("expected text content"),
+        }
+    }
+}