fix(session): defer text part creation until first delta

[wqymi]

Problem

The engine creates empty text parts at text-start time. When a model opens multiple text-starts but some have no actual content (empty text block before tool call), empty shell parts (len=0) are left in the database.

Fix

Defer text part creation to the first text-delta:

• text-start: only create in-memory currentText, no updatePart or stepPartIds.push
• text-delta: first delta triggers DB insert + stepPartIds registration (empty parts never created)
• text-end: empty text discarded (never persisted)
• cleanup: empty text discarded (never persisted)

Impact

• Empty text parts no longer exist in the database
• Consumers that read text parts (TUI, history reconstruction, etc.) are naturally compatible
• stepPartIds error cleanup path is safe (unpersisted ids are not in the list)

Tests

• typecheck passes
• max-mode tests pass
• copilot chat model tests pass
• auto-overflow tests pass