fix: harden regex matching on key:generate command

[paulbalandan]

Description

Hardens the regex used by spark key:generate to locate encryption.key in .env:

• A comment line that merely mentions encryption.key (e.g. # Note: encryption.key is set automatically by spark key:generate.) used to match the active pattern, so the command silently rewrote the comment instead of setting a real key. Comments are now matched by a separate pattern that uncomments them; comments that only mention the name are left intact and a fresh encryption.key = … line is appended.
• DotEnv supports export encryption.key = …. The previous regex didn't, so an existing export-prefixed key was missed and a duplicate appended. The new pattern matches the optional export prefix and preserves it on rewrite.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conform to PSR-12 coding style