test: add boundary condition and nil-input tests for command builder and MCP handlers

[jongio]

Summary

The buildCommand() function in cli/src/internal/executor/command_builder.go and several MCP handler functions lack tests for nil struct inputs, zero-value configurations, and boundary conditions. While callers currently validate inputs before calling these functions, the absence of defensive tests means refactoring could introduce nil-pointer panics.

Impact

• Nil pointer dereferences would crash the CLI or MCP server without a clear error message
• Boundary conditions (extremely long paths, special characters in shell names) are untested
• As the codebase grows, new callers may not replicate existing validation patterns

Specific Gaps

command_builder.go - buildCommand():

• No test for nil Config struct
• No test for empty shell name combined with failed OS detection
• No test for shell path containing spaces or special characters
• No test for extremely long script paths

mcp.go - handler functions:

• handleExecScript: no test for nil CallToolRequest or nil Arguments
• handleExecInline: no test for nil Arguments map
• handleGetEnvironment: no test with completely empty environment

executor.go - prepareEnvironment():

• No test for environment variable with = in the value (e.g., KEY=val=ue)
• No test for environment variable with empty key
• No test for duplicate environment variable keys

Recommendation

1. Add a TestBuildCommandBoundaryConditions table-driven test covering:

   • Nil/zero-value Config
   • Shell name with spaces
   • Very long script path (>260 chars on Windows)
   • Shell set to nonexistent binary

2. Add nil-input guards to MCP handler tests:

   • Nil request object
   • Request with nil Arguments map
   • Arguments with unexpected types (number where string expected)

3. Add environment edge case tests:

   • Values containing = characters
   • Empty string keys or values
   • Duplicate keys with different values