[test] Add tests for cmd.registerAllFlags, registerFlagCompletions, and OTLP flag defaults

[github-actions]

Test Coverage Improvement: cmd flags package

Function Analyzed

• Package: internal/cmd
• Primary Function: registerFlagCompletions
• Supporting Functions: registerAllFlags, RegisterFlag, getDefaultOTLPEndpoint, getDefaultOTLPServiceName
• Previous Coverage: 0% (none of these functions had any direct tests)
• Complexity: Medium-High (registerFlagCompletions registers 6 flag completions + ValidArgsFunction)

Why This Function?

registerFlagCompletions in flags.go was completely untested despite being a non-trivial function that configures shell completion behavior for 6 flags (--config, --log-dir, --payload-dir, --env, --guards-mode, --allowonly-min-integrity) plus a ValidArgsFunction. Every other flags_*.go file in the package had corresponding tests (e.g., flags_logging_test.go, flags_difc_test.go) but flags.go and flags_tracing.go had none.

Tests Added

internal/cmd/flags_tracing_test.go

• ✅ TestGetDefaultOTLPEndpoint: env var OTEL_EXPORTER_OTLP_ENDPOINT lookup (no env, custom value, empty → disabled, trailing slash)
• ✅ TestGetDefaultOTLPServiceName: env var OTEL_SERVICE_NAME with config.DefaultTracingServiceName fallback (5 cases)
• ✅ TestGetDefaultOTLPServiceName_DefaultIsCorrect: guards the "mcp-gateway" constant

internal/cmd/flags_test.go

• ✅ TestRegisterAllFlags: verifies all expected flags are registered (core, logging, tracing, DIFC), flagRegistrars is populated
• ✅ TestRegisterFlagCompletions: exercises all 6 flag completion functions and ValidArgsFunction:
  • --config → .toml extension filter
  • --log-dir / --payload-dir → directory filter
  • --env → .env extension filter
  • --guards-mode → {strict, filter, propagate} with no-file-comp
  • --allowonly-min-integrity → {none, unapproved, approved, merged} with no-file-comp
  • ValidArgsFunction → active help tip mentioning --config
• ✅ TestRegisterFlag: verifies RegisterFlag appends to flagRegistrars and all registrars are invoked

Test Pattern

Follows the same table-driven style as flags_logging_test.go and flags_difc_test.go. Uses t.Setenv for env var isolation and cobra.GetFlagCompletionFunc (available since cobra v1.6.1, project uses v1.10.2) to directly verify completion function registrations.

Coverage Report

Before: 0% direct test coverage for registerFlagCompletions, registerAllFlags, 
        getDefaultOTLPEndpoint, getDefaultOTLPServiceName
After:  All branches covered by 3 test functions (14 sub-tests total)

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