Use merge for county FIPS lookup

[trippusultan]

Closes #5804

Summary

Replaces indexed .loc lookups with a left pd.merge() when mapping county_fips to county_name and state in county formula.

Why

Issue #5804 requested moving to pd.merge() for clearer missing-value behavior and explicit join semantics.

Change

• File: policyengine_us/variables/household/demographic/geographic/county/county.py
• Replaced:
  • set_index(...).loc[...] lookups
• With:
  • pd.DataFrame({"county_fips": county_fips})
  • pd.merge(..., on="county_fips", how="left")
  • Read merged county_name and state columns

Notes

• Join preserves input order and yields NaN for unmatched FIPS rows.
• No behavior change intended for matched FIPS values.