Fcoalesce int double

[aksh08022006]

Fix fcoalesce Date / IDate handling and scalar safety

This PR fixes incorrect behavior and memory safety issues in fcoalesce() when
mixing Date and IDate inputs.

What was wrong

• fcoalesce(Sys.Date(), as.IDate(...)) errored due to incompatible storage
• Scalar fallbacks for logical / integer / numeric could return garbage values
• In some cases this led to incorrect results or segfaults in the test suite

What this PR does

• Normalizes any Date / IDate combination to Date with REAL storage
• Uses char_Date / char_IDate consistently (no invalid class assignment)
• Preserves existing semantics for non-date vectors
• Fixes scalar fallback handling to avoid uninitialized reads

Tests

• Added regression tests covering:
  • Date + IDate → Date
  • IDate NA fallback
  • Reversed argument order

All existing tests pass locally and in CI.

Closes: #7545

[jangorecki]

@aksh08022006 did you use AI when preparing this PR?

[aksh08022006]

@aksh08022006 did you use AI when preparing this PR?

yes

[jangorecki]

Please read https://github.com/Rdatatable/data.table/blob/master/.github/CONTRIBUTING.md#pull-requests-prs
Complete file, take a careful look at Pull requests section.

Next time before submitting PR be sure to test that package builds successfully and also pass R package check.