[codex] Add image re-encoding benchmarks (#23935)

## Summary
- add Divan benchmarks for prompt image re-encoding paths
- wire the image benchmark smoke test into Rust CI workflows

## Why
Image prompt handling includes re-encoding work that benefits from
repeatable benchmark coverage so changes can be measured in CI and
locally.

This already helped identify a potential regression from changing compiler flags.

## Impact
Developers can run and compare the new image re-encoding benchmarks, and
CI exercises the benchmark target via the Rust benchmark smoke test.

.github/workflows/rust-ci-full.yml

@@ -28,8 +28,13 @@ jobs:
       - uses: dtolnay/rust-toolchain@a0b273b48ed29de4470960879e8381ff45632f26 # 1.93.0
         with:
           components: rustfmt
+      - uses: taiki-e/install-action@44c6d64aa62cd779e873306675c7a58e86d6d532 # v2.62.49
+        with:
+          tool: just
       - name: cargo fmt
         run: cargo fmt -- --config imports_granularity=Item --check
+      - name: Rust benchmark smoke test
+        run: just bench-smoke
 
   cargo_shear:
     name: cargo shear

.github/workflows/rust-ci.yml

@@ -70,8 +70,13 @@ jobs:
       - uses: dtolnay/rust-toolchain@a0b273b48ed29de4470960879e8381ff45632f26 # 1.93.0
         with:
           components: rustfmt
+      - uses: taiki-e/install-action@44c6d64aa62cd779e873306675c7a58e86d6d532 # v2.62.49
+        with:
+          tool: just
       - name: cargo fmt
         run: cargo fmt -- --config imports_granularity=Item --check
+      - name: Rust benchmark smoke test
+        run: just bench-smoke
 
   cargo_shear:
     name: cargo shear