test: increase coverage and organize unit/integration suites

[vicajilau]

Summary

• increase coverage with new unit tests in SFTP, HTTP and channel message modules
• add protocol-level SFTP client tests using a local channel harness (no network)
• organize tests into unit and integration suites with tags and workflow split
• ignore generated coverage artifacts via .gitignore

Key Changes

• new tests:
  • test/src/sftp/sftp_packet_test.dart
  • test/src/sftp/sftp_packet_ext_test.dart
  • test/src/sftp/sftp_file_attrs_test.dart
  • test/src/sftp/sftp_name_test.dart
  • test/src/sftp/sftp_errors_test.dart
  • test/src/sftp/sftp_client_protocol_test.dart
  • test/src/http/http_client_test.dart
• expanded coverage in test/src/message/msg_channel_test.dart
• CI/workflow updates in .github/workflows/dart.yml
• test tag configuration in dart_test.yaml

Validation

• dart test passes locally (163 passed)
• coverage increased to ~55.83% overall

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 40.53%. Comparing base (8c151ea) to head (4750a0f).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #145      +/-   ##
==========================================
- Coverage   42.68%   40.53%   -2.15%     
==========================================
  Files          61       61              
  Lines        4791     4786       -5     
==========================================
- Hits         2045     1940     -105     
- Misses       2746     2846     +100     

Flag	Coverage Δ
unittests	40.53% <100.00%> (-2.15%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
lib/src/http/http_client.dart	27.05% <100.00%> (+27.05%)	⬆️
lib/src/sftp/sftp_packet.dart	48.55% <100.00%> (+44.54%)	⬆️

... and 30 files with indirect coverage changes

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