feat: Extend outbox processor with operational metrics (pulse.outbox.*)

[Copilot]

The outbox processor emitted no metrics, creating a blind spot for teams monitoring message delivery health in production.

New Metrics

Five instruments registered on the existing "NetEvolve.Pulse" meter:

Instrument	Type	Unit	Description
pulse.outbox.pending	ObservableGauge<long>	messages	Live pending count, sourced from IOutboxRepository
pulse.outbox.processed.total	Counter<long>	messages	Cumulative successfully processed
pulse.outbox.failed.total	Counter<long>	messages	Cumulative failed attempts
pulse.outbox.deadletter.total	Counter<long>	messages	Cumulative dead-lettered
pulse.outbox.processing.duration	Histogram<double>	ms	Elapsed time per polling cycle

Implementation

• IOutboxRepository — new default method GetPendingCountAsync() returning 0L; backward-compatible, no existing implementations need changes.
• OutboxProcessorHostedService — counters/histogram are static fields; the observable gauge is registered per-instance in the constructor, its callback reading a _pendingCount field updated via RefreshPendingCountAsync at the start of each cycle. All metric calls are wrapped in try/catch and log at Warning on failure — metric errors never interrupt message processing.
• Both individual (ProcessMessageAsync) and batch (ProcessBatchSendAsync) processing paths increment the appropriate counters.

// Pending count refreshed before each poll cycle, observed by the gauge callback
await RefreshPendingCountAsync(stoppingToken).ConfigureAwait(false);

var batchStartTime = Stopwatch.GetTimestamp();
var processedCount = await ProcessBatchAsync(stoppingToken).ConfigureAwait(false);
ProcessingDurationHistogram.Record(Stopwatch.GetElapsedTime(batchStartTime).TotalMilliseconds);

Tests

• 5 unit tests (one per instrument) using MeterListener to verify measurements are recorded.
• 2 integration tests verifying non-zero metrics after a full processing cycle.
• Test doubles updated to implement GetPendingCountAsync.

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[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 24 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.12%. Comparing base (19680ce) to head (48107c9).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...volve.Pulse/Outbox/OutboxProcessorHostedService.cs	67.60%	23 Missing ⚠️
...NetEvolve.Pulse.Extensibility/IOutboxRepository.cs	0.00%	1 Missing ⚠️

❌ Your patch status has failed because the patch coverage (66.66%) is below the target coverage (80.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #155      +/-   ##
==========================================
- Coverage   86.12%   85.12%   -1.01%     
==========================================
  Files          42       42              
  Lines        1312     1385      +73     
  Branches      123      125       +2     
==========================================
+ Hits         1130     1179      +49     
- Misses        145      169      +24     
  Partials       37       37              

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