fix(linter): add summary output to GitHub formatter

[Hideyasu-Ozawa]

Fixes #20402

Summary

• GitHub formatter produces no stdout when diagnostics are 0, unlike default and gitlab formatters
• This causes downstream tools that parse stdout (e.g. vite-plus) to fail in GitHub Actions
• Reuse get_diagnostic_result_output() from default.rs so finish() returns Found X warnings and Y errors. instead of None

Changes

File	Change
default.rs	Change get_diagnostic_result_output visibility to pub(super)
github.rs	finish() returns Some(get_diagnostic_result_output(result)) instead of None; update existing test + add new test
4 snapshot files	Add summary line to GitHub formatter snapshots

::warning / ::error annotations are unaffected. The summary line simply appears in the CI log.

Test plan

• Existing unit test updated (reporter_finish asserts summary content instead of None)
• New test reporter_finish_with_errors added for non-zero diagnostics
• All 4 GitHub formatter snapshot tests updated and passing
• Manual verification: GITHUB_ACTIONS=true ./target/debug/oxlint --format github outputs summary on 0 diagnostics
• just ready passes
• CI passes

This PR was developed with assistance from AI tools (Claude).
All changes have been reviewed and tested.

[Copilot]

Pull request overview

Updates oxlint’s GitHub Actions output formatter so it always emits a summary line even when there are zero diagnostics, aligning behavior with other formatters and preventing downstream tools from failing on empty stdout in CI.

Changes:

• Expose get_diagnostic_result_output to sibling formatters (pub(super)) for reuse.
• Make the GitHub reporter’s finish() return the standard summary string instead of None.
• Update/add unit + snapshot tests to assert the presence of the summary output for GitHub formatting.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
apps/oxlint/src/output_formatter/default.rs	Makes the shared summary-rendering helper available to sibling formatters.
apps/oxlint/src/output_formatter/github.rs	Ensures finish() always returns the standard summary output; updates/extends unit tests.
apps/oxlint/src/snapshots/fixtures__cli__output_formatter_diagnostic_--format=github ok.js@oxlint.snap	Snapshot now includes the summary line for the zero-diagnostics GitHub formatter run.
apps/oxlint/src/snapshots/fixtures__cli__output_formatter_diagnostic_--format=github test.js@oxlint.snap	Snapshot includes the summary line after GitHub annotations.
apps/oxlint/src/snapshots/fixtures__cli__output_formatter_diagnostic_--format=github parser-error.js@oxlint.snap	Snapshot includes the summary line for the parser-error GitHub formatter run.
apps/oxlint/src/snapshots/fixtures__cli__output_formatter_diagnostic_--format=github --report-unused-disable-directives disable-directive.js@oxlint.snap	Snapshot includes the summary line for unused-disable-directives output in GitHub format.

[camc314]

thanks