Add ToolNotFoundError for clear diagnostics when CLI tools are missing

[vrubezhny]

When a bundled CLI tool (e.g. oc) is not found or version-mismatched, executeSyncTool would pass undefined to execFileSync, causing a silent failure caught by requireLogin()'s .catch(() => true). The Explorer would show "Cluster is not accessible" with no indication that the real issue was a missing tool binary.

• Add ToolNotFoundError class in src/tools.ts with toolName property
• Guard executeSyncTool in cli.ts to throw ToolNotFoundError when ToolsConfig.detect() returns undefined
• Re-throw ToolNotFoundError in loginUtil.ts catch blocks so it propagates to callers instead of being silently swallowed
• Add unit tests for ToolNotFoundError class (tools.test.ts)
• Add unit tests for requireLogin() error propagation (loginUtil.test.ts)

Assisted-By: Claude Opus 4.6 [email protected]

[codecov]

Codecov Report

❌ Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.57%. Comparing base (da60441) to head (aebdbec).
⚠️ Report is 1887 commits behind head on main.

Files with missing lines	Patch %	Lines
src/cli.ts	66.66%	1 Missing ⚠️
src/util/loginUtil.ts	80.00%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #5968       +/-   ##
===========================================
+ Coverage   32.37%   50.57%   +18.20%     
===========================================
  Files          85      114       +29     
  Lines        6505    10388     +3883     
  Branches     1349     2334      +985     
===========================================
+ Hits         2106     5254     +3148     
- Misses       4399     5131      +732     
- Partials        0        3        +3     

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