test: add coverage for helper dispatch and service alias resolution

[Bortlesboat]

Summary

• Add unit tests for helpers::get_helper() — previously untested
• Expand services::resolve_service() test coverage with edge cases

New Tests

helpers/mod.rs

• test_get_helper_known_services — all 11 service names return a helper
• test_get_helper_unknown_returns_none — unknown/empty/wrong-case returns None
• test_get_helper_script_alias — both "script" and "apps-script" work
• test_helper_default_not_helper_only — default trait impl returns false

services.rs

• test_resolve_service_all_aliases — exhaustively tests every alias in SERVICES
• test_resolve_service_case_sensitive — "Drive" and "GMAIL" should fail
• test_resolve_service_empty_string — empty string should fail
• test_resolve_service_error_lists_known_services — error message includes available services
• test_no_duplicate_aliases — ensures no alias appears twice in SERVICES

Test plan

• cargo test passes (CI will verify)
• cargo clippy -- -D warnings clean (CI will verify)

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[changeset-bot]

🦋 Changeset detected

Latest commit: 9340f25

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@googleworkspace/cli	Patch

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[jpoehnelt]

test failure

[codecov]

Codecov Report

❌ Patch coverage is 98.36066% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 56.41%. Comparing base (473dd30) to head (9340f25).
⚠️ Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
src/services.rs	96.66%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #111      +/-   ##
==========================================
+ Coverage   56.21%   56.41%   +0.19%     
==========================================
  Files          38       38              
  Lines       13853    13914      +61     
==========================================
+ Hits         7788     7849      +61     
  Misses       6065     6065              

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