WP_Filesystem_Direct::getchmod() does not handle fileperms() errors

[vietcgi]

Description:

The getchmod() method in WP_Filesystem_Direct has a documented FIXME:

"FIXME does not handle errors in fileperms()"

When a file doesn't exist, fileperms() returns false, which gets converted to '0' - not a valid permission string.

This PR fixes it by returning false on failure, matching the pattern used by owner() and group() in the same class.

PR: ​https://github.com/WordPress/wordpress-develop/pull/10637

## Summary

The getchmod() method in WP_Filesystem_Direct had a documented FIXME noting it doesn't handle errors from fileperms().

When a file doesn't exist, fileperms() returns false, which then gets passed to decoct() and returns '0' - not a valid permission string.

This PR adds proper error handling to return false on failure, matching the pattern used by other methods in the class like owner() and group().

## Changes

• Return false when fileperms() fails
• Update return type to string|false
• Update test to verify false is returned for non-existent paths

## Testing

• Existing files: returns permissions as before (e.g. '644')
• Non-existent files: now returns false instead of '0'
• gethchmod() still works correctly (intval(false, 8) equals 0, same as before)

@westonruter it's looking good now. Thanks

[westonruter]

In 61398:

Filesystem API: Resolve FIXME comment for WP_Filesystem_Direct::getchmod() by explicitly returning '0' in error case.

Developed in ​https://github.com/WordPress/wordpress-develop/pull/10637

Follow-up to [11831].

Props vietcgi, westonruter.
See #10304.
Fixes #64426.