Fix input with inherited stdin

[puneetdixit200]

Fixes #1219

Summary

• Ignore a singular inherited stdin source when input or inputFile is provided, so explicit input becomes the only stdin source.
• Preserve explicit multi-source stdin arrays such as ['inherit', 'pipe'].
• Add nested regression coverage for stdin and stdio forms in async and sync modes.

Tests

• npm_config_cache=/tmp/execa-npm-cache npx ava test/io/input-option.js --match='*input option ignores*'
• npm_config_cache=/tmp/execa-npm-cache npx ava test/io/input-option.js test/stdio/native-inherit-pipe.js test/stdio/file-path-main.js
• npm_config_cache=/tmp/execa-npm-cache npm run lint
• npm_config_cache=/tmp/execa-npm-cache npm run type
• git diff --check

[ehmicky]

Hi @puneetdixit200,

Thanks a lot!
Perfect PR, nothing to fault! 👍

This is technically a breaking change, so would require a major release.
We should combine the next major release with dropping support for Node 18 and 20.
We should also fix #1233 (run tests on Node 26) as well.

To allow other non-breaking PRs to still be merged until we do the above, then we probably should wait to merge this PR, but mark it as ready to merge.
@sindresorhus Thoughts?