Guard remaining MD local tax variables against unknown counties

[MaxGhenis]

Extends #9116 (issue #8975) — same crash class, remaining instances.

Problem

md_applicable_local_tax_rate and md_flat_rate_county_tax carry the identical unguarded pattern that #9116 fixed in md_withheld_income_tax:

safe_county = where(in_md, county, "ALLEGANY_COUNTY_MD")   # guards non-MD only
flat_rate = p.flat_rate[safe_county]                       # UNKNOWN key -> crash

An MD household whose county resolves to County.UNKNOWN (the default when county_fips is unmapped/absent) raises ParameterNotFoundError: The parameter 'gov.local.md.flat_rate.UNKNOWN' was not found, crashing any computation of MD local income tax. The #8975 harness only hit the SALT-withholding chain, so these two siblings survived that report; verified pre-fix on this branch that the new UNKNOWN test case raises exactly that error.

Fix

Extend the same guard used by #9116 to both variables:

safe_county = where(in_md & (county != "UNKNOWN"), county, "ALLEGANY_COUNTY_MD")

No modeling-convention change: UNKNOWN falls back to the same default county the non-MD path already uses.

Tests

• New md_flat_rate_county_tax.yaml: Allegany baseline (3.03% × $30,000 = $909, hand-checked) + UNKNOWN case pinned equal to it (raised ParameterNotFoundError before the fix).
• md_applicable_local_tax_rate.yaml: Allegany baseline (0.0303) + UNKNOWN case pinned equal.
• Local run: MD local dir 19/19; full gov/states/md/tax/income 189/189.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (eea6512) to head (b711aa5).
⚠️ Report is 41 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #9147   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files            1         2    +1     
  Lines           18        42   +24     
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+ Hits            18        42   +24     

Flag	Coverage Δ
unittests	100.00% <100.00%> (ø)

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