Set placeholder for IntInput (Fixes #8359)

[emunsing]

1-liner fix to add a placeholder value to 8359. I'm not sure how to create a UI test for a placeholder value; happy to create one with help but given that this is a 1-line aesthetic change I don't know if that's needed.

[philippjfr]

Thanks @emunsing, I don't think we really need a UI test, a simply unit test checking that the underlying model inherits the correct placeholder value should be sufficient.

[philippjfr]

def test_int_input_placeholder(document, comm):

    int_input = IntInput(placeholder='Foo", name='IntInput')

    widget = int_input.get_root(document, comm=comm)

    assert widget.active == "Foo"
    
    int_input.placeholder = "Bar"
    assert widget.placeholder == "Bar"

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.00%. Comparing base (855143b) to head (1404713).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8360      +/-   ##
==========================================
- Coverage   86.09%   86.00%   -0.10%     
==========================================
  Files         349      349              
  Lines       54727    54734       +7     
==========================================
- Hits        47117    47073      -44     
- Misses       7610     7661      +51     

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[philippjfr]

Thanks @emunsing, wanted to make sure you get the credit so just added the test here instead of making a new PR.