ci: use selected files to upload results to codecov instead of file

[zimeg]

Summary

This PR changes the codecov step to use "files" for uploading test results. Per the kind migration guides:

🔗 https://github.com/codecov/codecov-action?tab=readme-ov-file#migration-guide

Notes

Fixes warnings of invalid input while perhaps still finding this file otherwise:

Warning: Unexpected input(s) 'file', valid inputs are ['base_sha', 'binary', 'codecov_yml_path', 'commit_parent', 'directory', 'disable_file_fixes', 'disable_search', 'disable_safe_directory', 'disable_telem', 'dry_run', 'env_vars', 'exclude', 'fail_ci_if_error', 'files', 'flags', 'force', 'git_service', 'gcov_args', 'gcov_executable', 'gcov_ignore', 'gcov_include', 'handle_no_reports_found', 'job_code', 'name', 'network_filter', 'network_prefix', 'os', 'override_branch', 'override_build', 'override_build_url', 'override_commit', 'override_pr', 'plugins', 'recurse_submodules', 'report_code', 'report_type', 'root_dir', 'run_command', 'skip_validation', 'slug', 'swift_project', 'token', 'url', 'use_legacy_upload_endpoint', 'use_oidc', 'use_pypi', 'verbose', 'version', 'working-directory']

🔗 https://github.com/slackapi/node-slack-sdk/actions/runs/19472021130/job/55721673209#step:12:1

Requirements

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.02%. Comparing base (90ed66e) to head (0d42854).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2439   +/-   ##
=======================================
  Coverage   93.02%   93.02%           
=======================================
  Files          40       40           
  Lines       11127    11127           
  Branches      713      713           
=======================================
  Hits        10351    10351           
  Misses        764      764           
  Partials       12       12           

Flag	Coverage Δ
cli-hooks	95.23% <ø> (ø)
cli-test	94.79% <ø> (ø)
oauth	77.39% <ø> (ø)
socket-mode	61.87% <ø> (ø)
web-api	98.08% <ø> (ø)
webhook	96.66% <ø> (ø)

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[WilliamBergamin]

Nice catch 💯

[zimeg]

@WilliamBergamin These warning logs were so interesting to find! Let's continue the quest to code health! 🏥 ✨