refactor(error): add typed error code enum to messaging_error_category (#220)

[kcenon]

Closes #220

Summary

• Add codes enum inside messaging_error_category that mirrors all 46 constexpr int error codes from error_codes.h
• Add make_typed_error_code(messaging_error_category::codes) overload for type-safe error code construction
• Add 7 new unit tests covering enum values, Result integration, and backward compatibility

Context

This completes the remaining phases of Issue #220:

• Phase 1 (error category definition): Already completed in PR [Refactor] Define messaging_error_category and typed error codes #229
• Phase 2 (typed error codes): Implemented here via codes enum
• Phase 3 (call site migration): Tracked separately in Issue [Refactor] Migrate make_error call sites to typed error codes #230
• Phase 4 (backward compatibility): Existing constexpr int codes and make_messaging_error_code() remain unchanged
• Phase 5 (testing): 7 new tests added (24 total in test file)

Design

Follows the same pattern as common_error_category::codes:

// Type-safe error code construction
auto ec = make_typed_error_code(messaging_error_category::queue_full);
return Result<int>::err(ec);

// Backward-compatible integer construction (still works)
auto ec2 = make_messaging_error_code(error::queue_full);

The codes enum uses unscoped enum codes : int (not enum class) so values are accessible directly as messaging_error_category::queue_full without the extra codes:: qualifier.

Test Plan

• All 24 messaging error category tests pass
• Full test suite passes (167/171, 4 pre-existing flaky failures)
• Enum values verified identical to constexpr int constants
• Enum-based and int-based construction produce equal typed_error_code objects
• Result integration works with both constructor and factory method
• Cross-category inequality verified

[kcenon]

CI/CD Failure Analysis

Analysis Time: 2026-02-14 UTC
Attempt: #1

Failed Workflows

Workflow	Job	Step	Status
CI	windows-2022 / msvc	Run tests (Windows)	Failed

Root Cause Analysis

Primary Error:

test_standalone_backend.cpp(163): error: Expected: (executor->worker_count()) > (0), actual: 0 vs 0
[  FAILED  ] StandaloneBackendTest.ZeroThreads (13 ms)

Analysis:
This is a flaky test unrelated to PR #235 changes. The failing test StandaloneBackendTest.ZeroThreads tests thread pool worker count in test_standalone_backend.cpp, while this PR only modifies:

• messaging_error_category.h (added codes enum)
• test_messaging_error_category.cpp (added 7 unit tests)

The worker_count() method calls get_active_worker_count(), which returns the number of actively running workers. When no tasks are queued, active count can be 0 — a classic race condition in thread pool tests.

Evidence this is pre-existing:

• Main branch CI (run #21991043695): windows-2022/msvc PASSED
• PR refactor(modules): update C++20 module files for upstream ecosystem changes (#223) #234 CI: windows-2022/msvc PASSED
• PR refactor(integration): integrate common_system health monitoring API (Issue #222) #233 CI: windows-2022/msvc PASSED
• PR refactor(adapters): replace local circuit breaker with common_system resilience module (Issue #221) #232 CI: windows-2022/msvc PASSED
• test_messaging_error_category (this PR's test): PASSED (0.02 sec)

Action Taken

Re-triggered the failed workflow. The flaky test should pass on retry.

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Automated failure analysis - Attempt #1