fix: validate unsafe file name characters

[tipogi]

Summary

• Adds validation for PubkyAppFile.name to reject characters unsafe for Content-Disposition headers.
• Rejects ", \, and control characters while preserving Unicode file names.
• Adds tests for valid names, invalid header-unsafe names, and offending-character error messages.

Test plan

• cargo test models::file::tests::test_validate_file_name_chars

Closes #131

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Export all characters rejected by file validation

Because VALIDATION_LIMITS is documented as the client-side source of truth, clients that consume fileNameInvalidChars will only reject " and \, while PubkyAppFile::validate also rejects every control character via c.is_control(). For names such as file\n.txt or file\0.txt, client-side validation built from these limits will pass data that the model then rejects, so either the exported limits need to include/represent the control-character rule or this field should not be presented as the full invalid-character set.

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