Clarify Merge Data Extra Data input labeling

[Pragati5-DEBUG]

Summary

• Renames the second Merge Data input from Extra Data to Extra Data (second table) so the connection menu makes the role clearer (replaces keeps old workflows working).
• Adds short input docs and an in-widget hint explaining Data = main table and Extra Data = second table.
• Shows an information message when only one of the two inputs is connected.

Motivation

New users often wire both links into Data, or are unsure what Extra Data means. Clearer naming and a short hint reduce that confusion without changing merge behavior.

Test plan

• Drag a link onto Merge Data and confirm the menu shows Extra Data (second table)
• Connect Datasets → Merge Data as Data, and a second source → Merge Data as Extra Data (second table)
• Open Merge Data and confirm the hint: Inputs: Data = main table · Extra Data = second table to merge
• Disconnect one input and confirm the info message appears; reconnect and confirm it clears
• Open an older workflow that used Extra Data and confirm it still loads

Screenshots

[CLAassistant]

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