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.`