fix: populate projected budget bill and deferred amount sensors

[CamiloValderruten]

Summary

Fixes #92 by addressing the KeyError and data fetching ordering when collecting Budget Billing details for enrolled accounts.

Root Cause

1. In FplMainRegionApiClient.update(), __getBBL_async() was executed before get_energy_usage().
2. Inside __getBBL_async(), projectedBillData["projected_bill"] threw a KeyError because projected bill data had not been fetched yet (and the key is stored as camelCase projectedBill).
3. The broad try...except caught the KeyError and returned an empty dictionary, skipping both the calculation of budget_billing_projected_bill / budget_billing_daily_avg and the subsequent call to URL_BUDGET_BILLING_GRAPH (which populates defered_amount).

Changes

• Reordered Account Update: Fetch get_energy_usage() and get_appliance_usage() prior to evaluating BBL program and running __getBBL_async().
• Safe Dictionary Lookups: Used .get() for projectedBill and as_of_days in __getBBL_async().
• Isolated HTTP Requests: Separated URL_BUDGET_BILLING_PREMISE_DETAILS and URL_BUDGET_BILLING_GRAPH into dedicated try...except blocks with clear error logging.
• Defensive Fallback: Handled None checks in BillToDateSensor.
• Unit Tests: Added tests/test_budget_billing.py covering BBL API client calculations, ordering in update(), partial failure resilience, and sensor state resolutions.

Verification

Ran test suite:

.venv/bin/python -m unittest discover tests

All 10 tests passed.