fix(dnn): update int8-missing test to reflect ADR-1032 fp32-fallback

[lusoris]

Summary

• test_session_open_int8_missing_returns_error was asserting rc < 0 when .int8.onnx is absent, but PR fix(libvmaf,dnn): vmaf_init double-init guard + vmaf_close pointer-zero + DNN fallback path #642 (ADR-1032) intentionally changed vmaf_dnn_session_open to fall back to the fp32 baseline instead of returning an error.
• The test was not updated to match the new policy, causing libvmaf:dnn / test_dnn_session_api to fail in CI.
• Rename the test to test_session_open_int8_missing_falls_back_to_fp32 and flip the assertions to verify the fp32 fallback path: rc == 0, session pointer populated, session usable.

Root cause

Commit 1958126bb (PR #642, ADR-1032) changed the behavior in core/src/dnn/dnn_api.c (lines 108–119): on missing .int8.onnx, rc is now set to 0 and execution falls through to vmaf_ort_open with the original fp32 path. The test file was not updated in that PR.

Reproducer

meson setup core/build -Denable_dnn=true -Denable_cuda=false -Denable_sycl=false
ninja -C core/build test/dnn/test_dnn_session_api
meson test -C core/build --suite=dnn --print-errorlogs
# Before fix: 6/12 test_dnn_session_api FAIL (exit status 1)
# After fix:  6/12 test_dnn_session_api OK

Checklist

• no rebase impact: pure test-only fix, no public API touched
• no digest needed: trivial — one-line behavioral policy already captured in ADR-1032
• no alternatives: only-one-way fix — test must match the production behavior
• no rebase-sensitive invariants
• changelog.d/ fragment: not required for test-only fixes with no user-visible delta (ADR-0108 §exclusions)
• docs/state.md: not required — this is a test fix, not a bug open/close against production behavior

🤖 Generated with Claude Code

[lusoris]

Closing in favor of rebased version (conflict resolved by cherry-pick onto current master).