fix(files_sharing): Restore password guard return for new public shares

fix(files_sharing): Restore password guard return for new public shares

Creating a new public share without a password silently succeeded: the

password error was shown but execution continued, and the share was

created without a password. Users had to save a second time for the

password to apply.

The password guard now blocks the save when a new public share is

missing a password. Non-public shares (user/group) are never blocked

by this guard, so they remain unaffected.

Tests invoke the real saveShare method against a stubbed context and

cover the save-twice symptom and the non-public-share regression.

Signed-off-by: nfebe <fenn25.fn@gmail.com>

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Author: nfebe