[voropaevv]

When SDK client-side downsizing is enabled (client_downsizing_disabled=False), resizing a Pillow PA image can lose its palette colors before JPEG encoding. An opaque input with RGB color (64, 128, 192) produces a black uploaded JPEG.

Expand PA to RGBA before an actual resize, after the no-resize guards. This uses Pillow's existing alpha-aware resampling and preserves the source image, scaling factor, other image modes and JPEG encoding policy.

The 25 regression cases cover opaque/mixed/zero alpha, nonmonotonic palettes, no-resize behavior, other modes, and actual synchronous/asynchronous SDK request bytes with prediction geometry.

Validation on 30141d2f6 against main 21059e9a5:

• New tests: 8 failed / 17 passed on the parent, then 25 passed with the fix on both Pillow 12.3 and 13.
• Full SDK unit suite: 696 passed / 1 existing filesystem skip on each version.
• The 25 new cases also pass on Python 3.10 and 3.13 and against the built wheel installed into an isolated target using the existing test dependencies.
• Regression tests reject RGB-only conversion and conversion before the no-resize guards.
• make check_code_quality, changed-file Black/isort/Flake8 and git diff --check pass.

The existing skip concerns case-distinct filenames on this filesystem. The all-catalog SDK run additionally has two WebRTC e2e setup errors because the SDK-only environment lacks torch; the same fixture errors reproduce on the parent before server startup. No model/server/GPU inference or hosted CI result is claimed here.

The declared Pillow dependency is unchanged; Pillow 13 is extra runtime compatibility evidence. Thin-image rounding in #3241 is outside this change.