test: improve test coverage

[ncode]

Summary

Add comprehensive tests for previously uncovered code paths:

Wrapper tests:

• consulClient wrapper methods (Agent, Catalog, Health, Session, KV)
• commandExecutor.CommandContext

Async execution tests:

• updateServiceTags command execution (ExecOnPromote/ExecOnDemote)
• Error handling in command execution goroutine
• Session data retrieval failures

Edge case tests:

• Run with small TTL (< 2s interval branch)
• Run election error handling in ticker loop
• updateLeadershipStatus error return path
• New viper unmarshal error
• New default values for LockDelay/TTL/Name

Coverage

Local measurements:

• New: 85.7% → 96.4%
• Run: 69.2% → 76.9%
• updateLeadershipStatus: 80% → 100%
• consulClient wrappers: 0% → 100%
• commandExecutor.CommandContext: 0% → 100%

Note: HealthWrapper.Checks remains at 0% - it wraps *api.Health (concrete type) and requires a real Consul instance (covered by integration tests).

Test plan

• All existing tests pass
• New tests pass
• All modified and coverable lines are covered

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.56%. Comparing base (cadac98) to head (29afeb6).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #28      +/-   ##
==========================================
+ Coverage   78.25%   86.56%   +8.31%     
==========================================
  Files           4        4              
  Lines         469      469              
==========================================
+ Hits          367      406      +39     
+ Misses         82       45      -37     
+ Partials       20       18       -2     

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