test: improve interactive shell completion coverage

[timvw]

Summary

• add positive interactive completion tests for zsh and bash covering command, branch, and config subcommand completion flows
• add Windows-only PowerShell completion tests using CommandCompletion API for command, branch, and config-subcommand cases
• reduce interactive test flakiness and runtime by removing premature output-stability aborts in wait logic and building the wt test binary once per run

Validation

• go test ./...
• go test ./... -run 'TestZshTabCompletion|TestBashTabCompletion|TestPowerShellCompletion' -v

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 27.38%. Comparing base (661d0f9) to head (15ae000).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #54      +/-   ##
==========================================
+ Coverage   27.25%   27.38%   +0.12%     
==========================================
  Files           4        4              
  Lines        1574     1574              
==========================================
+ Hits          429      431       +2     
+ Misses       1095     1094       -1     
+ Partials       50       49       -1     

see 1 file with indirect coverage changes

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