Add comprehensive test coverage for Lambda Labs error handling functions

Summary

This PR adds comprehensive test coverage for the previously untested error handling functions in the Lambda Labs implementation: handleAPIError and handleErrToCloudErr. The new test suite covers various HTTP error scenarios, API error transformations, and edge cases to ensure robust error handling behavior.

Key Changes:

• Created internal/lambdalabs/v1/errors_test.go with 13 test functions covering both error handling functions
• Added tests for different HTTP status codes (4xx, 5xx), rate limiting, instance not found scenarios
• Implemented reflection-based testing for private openapi.GenericOpenAPIError fields
• Added comprehensive coverage for error transformation logic (capacity errors → ErrInsufficientResources, region errors, etc.)
• Updated go.mod/go.sum with test dependencies

Review & Testing Checklist for Human

• Run tests locally - Verify go test ./internal/lambdalabs/v1 -v passes and all error test cases work as expected
• Review reflection usage - Check if the reflection approach for testing openapi.GenericOpenAPIError private fields is appropriate and won't break with future changes
• Validate error message patterns - Confirm that the string matching logic in tests aligns with actual Lambda Labs API error responses (especially capacity and region error messages)
• Verify dependency changes - Ensure go.mod/go.sum updates are legitimate and don't introduce unwanted dependencies
• Consider missing scenarios - Review if there are additional error cases from Lambda Labs API that should be tested

Recommended Test Plan: Run the full test suite locally and verify that the error handling behaves correctly for typical Lambda Labs API failure scenarios.

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    errors_go["internal/lambdalabs/v1/<br/>errors.go"]:::context
    errors_test_go["internal/lambdalabs/v1/<br/>errors_test.go"]:::major-edit
    pkg_errors_go["pkg/v1/<br/>errors.go"]:::context
    openapi_client["internal/lambdalabs/gen/<br/>lambdalabs/client.go"]:::context
    go_mod["go.mod"]:::minor-edit
    go_sum["go.sum"]:::minor-edit

    errors_test_go -->|"tests"| errors_go
    errors_test_go -->|"imports error types"| pkg_errors_go
    errors_test_go -->|"uses GenericOpenAPIError<br/>with reflection"| openapi_client
    errors_test_go -->|"test dependencies"| go_mod
    go_mod -->|"dependency lock"| go_sum

    subgraph Legend
        L1[Major Edit]:::major-edit
        L2[Minor Edit]:::minor-edit
        L3[Context/No Edit]:::context
    end

classDef major-edit fill:#90EE90
classDef minor-edit fill:#87CEEB
classDef context fill:#FFFFFF

Notes

• The reflection-based approach for testing GenericOpenAPIError is necessary because the struct has private fields, but it's brittle and should be monitored for future OpenAPI client updates
• Error message matching is case-sensitive and uses substring matching, which worked for the test cases but may need adjustment if Lambda Labs changes their error message formats
• All tests follow existing patterns from other test files in the codebase using testify and table-driven tests

Link to Devin run: https://app.devin.ai/sessions/ff49c2c138e847398cb946898211a43f
Requested by: Alec Fong (@theFong)