Fix: apply inspect-based kwarg filtering to start_session() to fix multiplex predictor TypeError

[kuihao]

Problem

When using SAM 3.1 with build_sam3_multiplex_video_predictor, calling handle_request(request=dict(type="start_session", ...)) raises:

TypeError: Sam3MultiplexTrackingWithInteractivity.init_state() 
got an unexpected keyword argument 'offload_state_to_cpu'

This happens because start_session() unconditionally passes
offload_state_to_cpu to self.model.init_state(), but
Sam3MultiplexTrackingWithInteractivity.init_state() does not accept
this parameter.

Root Cause

sam3_base_predictor.py already uses inspect.signature to filter
kwargs in add_prompt() and propagate_in_video(), but start_session()
was missing the same guard.

Fix

Apply the same inspect-based filtering pattern already used in this
file to start_session(), so only parameters accepted by the underlying
model's init_state() are forwarded.

Reproduction

Run the official sam3.1_video_predictor_example.ipynb with a
multiplex predictor — the start_session call on cell [11] raises
the TypeError above.

Testing

Ran the existing test suite to confirm no regressions:

$ python -m pytest test/test_io_utils.py -v
============================================================== test session starts ==============================================================
platform linux -- Python 3.12.3, pytest-8.1.1, pluggy-1.6.0
rootdir: /home/kuihao/Git/sam3
configfile: pyproject.toml
plugins: cov-7.1.0, hydra-core-1.3.2, anyio-4.9.0, flakefinder-1.1.0, xdoctest-1.0.2, shard-0.1.2, xdist-3.6.1, hypothesis-6.130.8, rerunfailures-15.1, typeguard-4.3.0
collected 8 items                                                                                                                               
Running 8 items in this shard

test/test_io_utils.py ........                                                                                                            [100%]

=============================================================== warnings summary ================================================================
sam3/model_builder.py:8
  /home/kuihao/Git/sam3/sam3/model_builder.py:8: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
    import pkg_resources

../../../../usr/local/lib/python3.12/dist-packages/timm/models/layers/__init__.py:49
  /usr/local/lib/python3.12/dist-packages/timm/models/layers/__init__.py:49: FutureWarning: Importing from timm.models.layers is deprecated, please import via timm.layers
    warnings.warn(f"Importing from {__name__} is deprecated, please import via timm.layers", FutureWarning)

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
========================================================= 8 passed, 2 warnings in 1.64s =========================================================

No existing unit test covers start_session() kwarg filtering.
The fix was verified end-to-end via sam3.1_video_predictor_example.ipynb
with a multiplex predictor — cell [11] no longer raises TypeError.

Happy to add a dedicated unit test if maintainers can point to
the preferred location for predictor-layer test coverage.

Fixes #544