fix: default hyperparams to float32 unless specified

[akramcodez]

Respect hyperparam_dtype in inject_hyperparams.

If hyperparam_dtype is not provided, injected hyperparameters now default to jnp.float32 rather than inheriting dtypes from parameters/updates. Explicit hyperparam_dtype values continue to be honored. This avoids accidental low-precision hyperparameters (e.g., bfloat16) while preserving explicit control.

• Files: _inject.py
• Tests: Local suite passed + targeted schedule tests passed. Also verified via small reproducer: default → float32; explicit hyperparam_dtype=jnp.bfloat16 → bfloat16.
• Note: A unit test for default dtype is prepared but omitted per instruction; can include if requested.

Fixes: #1414

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[akramcodez]

PTAL @sizhit2