ci: replace deprecated codspeed instrumentation mode with simulation (#36047)

- Replace deprecated `instrumentation` runner mode with `simulation` in
CodSpeed benchmarks (purely a rename, identical behavior)
- Scope partner benchmarks to `tests/unit_tests/ -m benchmark` instead
of `tests/` — avoids collecting ~80-90 non-benchmark tests per partner
just to deselect them
- Integration benchmark (`test_stream_time`) is always skipped anyway
(no VCR cassettes in CI), so no loss from excluding `integration_tests/`

Resolves: `Warning: The 'instrumentation' runner mode is deprecated and
will be removed in a future version. Please use 'simulation' instead.`

.github/workflows/check_diffs.yml

@@ -234,9 +234,9 @@ jobs:
             if [ "${{ matrix.job-configs.working-directory }}" = "libs/core" ]; then
               uv run --no-sync pytest ./tests/benchmarks --codspeed
             else
-              uv run --no-sync pytest ./tests/ --codspeed
+              uv run --no-sync pytest ./tests/unit_tests/ -m benchmark --codspeed
             fi
-          mode: ${{ matrix.job-configs.working-directory == 'libs/core' && 'walltime' || 'instrumentation' }}
+          mode: ${{ matrix.job-configs.working-directory == 'libs/core' && 'walltime' || 'simulation' }}
 
   # Final status check - ensures all required jobs passed before allowing merge
   ci_success: