Add CLI integration tests using a mock stellar binary to test argument parsing, error handling, and output formatting

[ALLEN-AYODEJI]

Description

The CLI has zero automated tests. Every command shells out to the stellar binary, making tests dependent on external infrastructure. A mock stellar binary that returns controlled responses would enable comprehensive CLI testing: argument parsing, error handling, output formatting, and JSON serialization.

Requirements and context

• Security: Standard security review required for state-mutating changes
• Testing: Full test suite must pass; new tests for changed behavior
• Documentation: Update doc comments and README if user-facing

Suggested execution

Branch: fix/feat/test-cli-mock

Implement changes

• Create a test_stellar.sh/bat script that simulates stellar CLI responses based on arguments
• Add a test environment variable (TRELLIS_TEST_MODE=true) that points to the mock binary
• Write tests for each CLI command: verify correct stellar invocation, verify output parsing
• Add error-path tests: stellar returns error, stellar not found, stellar returns invalid JSON

Test and commit

Run CLI tests with mock binary. Verify all 8 commands are tested for success and error paths. Run cargo test.

Example commit message

feat(test): add CLI unit tests with mock stellar binary for comprehensive argument and error path testing

Guidelines

• Open a PR against master
• Keep PR focused on this single concern
• Run cargo test or npm test before review
• Ensure no lint or typecheck warnings