Add report-output validation & `GitHubRelease` tests

[samatstariongroup]

Prerequisites

• I have written a descriptive issue title
• I have verified that I am running the latest version of the EcoreNetto
• I have searched open and closed issues to ensure it has not already been reported

Description

• Summary: Report-generator tests are smoke tests that don't validate output structure, and the GitHubRelease service / VersionChecker error paths are not covered.
• Affected files: ECoreNetto.Reporting.Tests/Generators/**, ECoreNetto.Tools.Tests/**; subjects in ECoreNetto.Reporting/Generators/** and ECoreNetto.Tools/Services/**.
• Proposed solution: Assert key structural content of generated HTML/Markdown/Excel output (ties into A2), and test version-check success/failure/timeout with a mocked HTTP handler (ties into A5).
• Acceptance criteria:
  • Generator tests assert specific output content, not just "a file was produced".
  • VersionChecker/GitHubRelease success and failure paths are tested with a mocked HTTP layer.