Scale parameter thresholds don't support uprating

[MaxGhenis]

Issue Description

Scale parameters (marginal_rate type) don't properly support uprating for thresholds. Neither the threshold_uprating field in the scale metadata nor the uprating field in individual threshold metadata work to extend threshold values beyond explicitly defined years.

Current Behavior

When defining a scale parameter with thresholds that should be uprated, the thresholds remain fixed at the last defined value instead of being uprated for future years.

Expected Behavior

Thresholds in scale parameters should be uprated according to the specified uprating parameter, similar to how simple parameters work.

Example

# This doesn't work - thresholds stay at 2025 values for 2030+
description: PIA formula factors
metadata:
  type: marginal_rate
  threshold_unit: currency-USD
  rate_unit: /1
  threshold_uprating: gov.ssa.uprating  # Doesn't work
brackets:
  - threshold:
      2024-01-01: 1_174
      2025-01-01: 1_226
    rate:
      1979-01-01: 0.32
    metadata:
      threshold:
        uprating: gov.ssa.uprating  # Also doesn't work

Use Case

Social Security PIA calculation uses bend points that are indexed to wage growth. Tax brackets and many other scale-based calculations also need threshold uprating.

Related Issues

• Option to only uprate thresholds or amounts in scale parameters #163 discusses selective uprating but doesn't address the core issue that threshold uprating doesn't work at all

Proposed Solution

Support uprating for scale thresholds through either:

1. threshold_uprating in the scale metadata (applies to all thresholds)
2. uprating in individual threshold metadata (per-threshold control)
3. Both (with individual metadata overriding scale-level default)

This would match the behavior of simple parameters and amount uprating in scales.