test: bound learning rate scheduling example

[kholdrex]

Summary

Bounds the learning-rate scheduling example so it stays quick and deterministic for interactive use and regression coverage. The example now uses explicit demo-budget constants and test-visible configuration helpers for each scheduler path.

Changes

• Adds bounded train/validation fixture sizes, sequence length, hidden sizes, epoch budgets, and scheduler period constants.
• Updates the learning-rate scheduling demos to use the shared bounded configuration helpers.
• Adds regression tests that verify every learning-rate scheduling demo path remains bounded and that generated sine-wave fixtures are deterministic.

Test plan

• cargo test --test example_training_bounds_test learning_rate_scheduling --all-features
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings
• cargo test --all-features