Add nan parameter to fcoalesce for NaN/NA distinction control

[Mukulyadav2004]

fixes #4567

Currently fcoalesce always treats NaN as missing (same as NA_real_), providing no user control over NaN handling. This inconsistency with nafill, which offers a nan parameter for controlling whether NaN should be treated distinctly from NA.

This PR adds a nan parameter to fcoalesce with default behavior nan=NA (treating NaN as missing, default) and option nan=NaN (preserving NaN values), ensuring fcoalesce(x, y, nan=Z) behaves consistently with nafill(x, fill=y, nan=Z) when y is length-1.

Implemetation:
In wraper.R - Added nan parameter with default behaviour - nan = NA means treat NaN as NA - in both fcoalesce as well setcoalesce
Then update header - data.table.h to match new function signature.

In coalesce.c

• Add third parameter - nan_is_na_arg to support user control
• After then extract and validate the nan control parameter
• Apply user's nan preference when checking scalar fill values, for the final value checking and in main replacement loop.

The logic is :
nan_is_na(NA) -> TRUE - then use ISNAN(val) - thus both NaN and NA_real treated as missing
nan_is_na(NaN) -> FALSE - then use ISNA(val) - thus only NA_real treated as missing, NaN preserved

In Docs change - coalesce.Rd :

• Added nan parameter to both function signatures
• Added explanation of how nan = NA vs nan = NaN affects behaviour.
• Demonstrate the difference between default and explicit nan = NaN behaviour

@MichaelChirico @jangorecki can you please review

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.77%. Comparing base (2b191ae) to head (2ce965f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7189   +/-   ##
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  Coverage   98.77%   98.77%           
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  Files          81       81           
  Lines       15204    15215   +11     
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+ Hits        15018    15029   +11     
  Misses        186      186           

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[github-actions]

• HEAD=issue_4567 slower P<0.001 for DT[by,verbose=TRUE] improved in #6296
  

Generated via commit 2ce965f

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	3 minutes and 3 seconds
Installing different package versions	41 seconds
Running and plotting the test cases	2 minutes and 38 seconds

[MichaelChirico]

Suggested change

	fcoalesce(\dots, nan = NA)
	fcoalesce(\dots, nan=NA)

[MichaelChirico]

LGTM! Please add a NEWS entry

[MichaelChirico]

sorry, last step is to merge master to resolve the conflict

[Mukulyadav2004]

Done, sorry for the delay, my laptop's been quite slow but the merge is complete now.