Route image extension reads through turn environments v2 (#27498)

## Why

Image generation used `std::fs::read` for referenced image paths, which
did not support environment-backed filesystems or their sandbox context.

## What changed

- Expose optional turn environments to extension tool calls.
- Include each environment’s ID, working directory, filesystem, and
sandbox context.
- Read referenced images through the selected environment filesystem.
- Keep sandbox usage at the extension call site so extensions can choose
the appropriate access mode.
- Consolidate image request construction into one async function.
- Add coverage for successful environment reads and read failures.

## Validation

- `cargo check -p codex-image-generation-extension --tests`
- `just fmt`
- `just bazel-lock-update`
- `just bazel-lock-check`

`just test -p codex-image-generation-extension` could not complete
because the build exhausted available disk space.

codex-rs/ext/memories/src/tests.rs

@@ -215,6 +215,7 @@ async fn add_ad_hoc_note_tool_creates_note_file() {
             truncation_policy: TruncationPolicy::Bytes(1024),
             conversation_history: codex_extension_api::ConversationHistory::default(),
             turn_item_emitter: Arc::new(NoopTurnItemEmitter),
+            environments: Vec::new(),
             payload: payload.clone(),
         })
         .await
@@ -258,6 +259,7 @@ async fn add_ad_hoc_note_tool_rejects_paths_as_filenames() {
             truncation_policy: TruncationPolicy::Bytes(1024),
             conversation_history: codex_extension_api::ConversationHistory::default(),
             turn_item_emitter: Arc::new(NoopTurnItemEmitter),
+            environments: Vec::new(),
             payload,
         })
         .await;
@@ -302,6 +304,7 @@ async fn read_tool_reads_memory_file() {
             truncation_policy: TruncationPolicy::Bytes(1024),
             conversation_history: codex_extension_api::ConversationHistory::default(),
             turn_item_emitter: Arc::new(NoopTurnItemEmitter),
+            environments: Vec::new(),
             payload: payload.clone(),
         })
         .await
@@ -349,6 +352,7 @@ async fn search_tool_accepts_multiple_queries() {
             truncation_policy: TruncationPolicy::Bytes(1024),
             conversation_history: codex_extension_api::ConversationHistory::default(),
             turn_item_emitter: Arc::new(NoopTurnItemEmitter),
+            environments: Vec::new(),
             payload: payload.clone(),
         })
         .await
@@ -422,6 +426,7 @@ async fn search_tool_accepts_windowed_all_match_mode() {
             truncation_policy: TruncationPolicy::Bytes(1024),
             conversation_history: codex_extension_api::ConversationHistory::default(),
             turn_item_emitter: Arc::new(NoopTurnItemEmitter),
+            environments: Vec::new(),
             payload: payload.clone(),
         })
         .await
@@ -475,6 +480,7 @@ async fn search_tool_rejects_legacy_single_query() {
             truncation_policy: TruncationPolicy::Bytes(1024),
             conversation_history: codex_extension_api::ConversationHistory::default(),
             turn_item_emitter: Arc::new(NoopTurnItemEmitter),
+            environments: Vec::new(),
             payload,
         })
         .await;