fix: rescale decimal results in floor and ceil to scale 0

[skushagra]

Which issue does this PR close?

• Closes FLOOR decimal returns unexpected error #20640

Which issue does this PR fix?

Fixes FLOOR('-999.9'::DECIMAL(4,1)) producing a "Decimal overflow" error instead of returning -1000. Same issue affected CEIL for the positive boundary case.

What changes are included in this PR?

• decimal.rs — Modified apply_decimal_op to:
  • Set output type to Decimal(p, 0).
  • Rescale the result to scale 0.
  • Validate the rescaled value against Decimal(p, 0)
• floor.rs — Updated return_type to return Decimal(p, 0) for all decimal inputs
• ceil.rs — Same return_type change as floor

Are these changes tested?

Yes

Are there any user-facing changes?

Yes. This changes the behaviour of the floor and ceil functions

[skushagra]

It seems some tests require the scale to be preserved. Ill look into this and update.

[skushagra]

@MichaelScofield can I get some help here. I can update the testcases to accept the new behaviour, but is this new design really required?