fix: change vision mixer num_inputs from enum to uint

[srperens]

Summary

• Changed vision mixer num_inputs property from Enum (restricted to 2,4,6,8,10) to UInt allowing any value 2-10
• Backend already clamps to valid range and layout code handles arbitrary input counts
• Removes artificial even-number restriction from the UI

Test plan

• Create a vision mixer block with odd input counts (3, 5, 7, 9) and verify pipeline builds correctly
• Verify multiview layout renders correctly with odd input counts
• Verify existing even input counts still work as before

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