Fix 'write_session' to handle new subjects

[sadhana-r]

When writing a session to database, the session directory should be created if it doesn't already exist to handle situations where a new subject has been created.

• Bug fix to create session directory if it doesn't exist
• Write unit test

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Traceability Information:

• Issue: OpenLIFU-python#137
• SOFTREQ: OpenLIFU-python#56
• Comment: This issue is related to the requirements defined in OpenLIFU-python#56.

This information was populated automatically by a script on 2025-06-09T18:01:42Z.

[ebrahimebrahim]

This is kind of related to #124 also

[sadhana-r]

Oh I can address that in this PR then.