docs(samples): Created session_sample.py

[holtskinner]

This code was written by Gemini ✨

[snippet-bot[bot]]

Here is the summary of changes.

You are about to add 5 region tags.

• discoveryengine/session_sample.py:19, tag genappbuilder_create_session
• discoveryengine/session_sample.py:57, tag genappbuilder_get_session
• discoveryengine/session_sample.py:90, tag genappbuilder_delete_session
• discoveryengine/session_sample.py:122, tag genappbuilder_update_session
• discoveryengine/session_sample.py:164, tag genappbuilder_list_sessions

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