Fix sshAgent casacading calls with QSignalBlocker

[AlexpFr]

fix: block SSH agent signals on programmatic UI updates

Block attachmentComboBox/externalFileEdit/m_attachments.data() signals from emitting when changed by code (not user interaction). Uses QSignalBlocker pattern.

Functions affected:

• setForm()
• clear()
• setSSHAgentSetting()
• updateSSHAgentAttachments()

These modify UI elements, trigger unwanted signals, and recursively call:

• updateSSHAgentKeyInfo()
• updateSSHAgentAttachment()
• updateSSHAgentAttachments()

Implementation:
Add QSignalBlocker on programmatic changes:

QSignalBlocker sshAgent_attachmentComboBox_Blocker(m_sshAgentUi->attachmentComboBox);
QSignalBlocker sshAgent_externalFileEdit_Blocker(m_sshAgentUi->externalFileEdit);
QSignalBlocker attachmentsBlocker(m_attachments.data());

Testing strategy

• Compile with -DWITH_XC_SSHAGENT=ON
• Trace signals and function calls when opening/modifying/closing entries
• See debug branch: fix/sshAgent-cascading-calls-QSignalBlocker-debug

Type of change

• ✅ Bug fix (non-breaking change that fixes an issue)