add_boundary leaves params/param_bounds inconsistent and validate_config passes it

ModelConfigBuilder.add_boundary records the new boundary's parameters in
boundary_params, but does not add them to params, param_bounds or
n_params. The resulting config simulates fine — the boundary parameter is
routed through boundary_params — but any consumer that reads the parameter box
never sees it.

validate_config(strict=True) reports the config as valid.

Reproducer

import numpy as np
from ssms.config import register_boundary
from ssms.config.model_config_builder import ModelConfigBuilder

def exp_collapse(t, a=1.0, rate=0.5):
    import numpy as np
    return a * np.exp(-rate * np.asarray(t))

register_boundary("exp_collapse", exp_collapse, ["a", "rate"])
cfg = ModelConfigBuilder.from_model("ddm")
cfg = ModelConfigBuilder.add_boundary(cfg, "exp_collapse", ["a", "rate"])

print("params         :", cfg["params"])
print("n_params       :", cfg["n_params"])
print("param_bounds w :", len(cfg["param_bounds"][0]))
print("boundary_params:", cfg["boundary_params"])
print("validate_config(strict=True) ->", ModelConfigBuilder.validate_config(cfg, strict=True))

params         : ['v', 'a', 'z', 't']          <- 'rate' absent
n_params       : 4                             <- still 4
param_bounds w : 4                             <- still 4
boundary_params: ['a', 'rate']
validate_config(strict=True) -> (True, [])

Why it matters

Simulator(model=cfg).simulate(theta={..., "rate": 0.7}) works, so this is easy
to miss. But param_bounds is what downstream code uses to derive priors and
LAN training ranges, so a boundary parameter added this way silently has no
bounds and no declared existence.

Possible fixes

Either have add_boundary extend params / param_bounds / n_params with the
new (non-a) boundary parameters, or have validate_config flag boundary/drift
parameters that are missing from params. The current combination — inconsistent
config that the validator passes — seems like the one option to avoid.

Version: ssm-simulators 0.13.2

Start in ssms/config/model_config_builder.py at ModelConfigBuilder.add_boundary and validate_config, then run the supplied reproducer with the ssm-simulators 0.13.2 configuration. Trace how boundary_params, params, param_bounds, and n_params are assembled. Done means boundary parameters are represented consistently and strict validation no longer accepts a missing parameter declaration.