Introduce datatable.unique.names policy for duplicate handling in setnames() #4044

[venom1204]

closes #4044
This PR introduces a configurable policy for handling duplicate column names created by setnames().

Changes introduced:

Added a new global option datatable.unique.names (default: "off") to preserve backward compatibility.

Supported policies:

• "off" – Allow duplicates silently (current behavior).
• "warn" – Issue a warning when duplicates are created.
• "error" – Stop execution if duplicates would be created.
• "rename" – Automatically enforce uniqueness using make.unique().

Added a centralized helper process_name_policy() in utils.R to handle duplicate detection and enforcement.

Integrated the policy check into setnames() before reference updates to ensure keys and indices are not corrupted in "error" or "rename" modes.

• Added validation for invalid option values with a warning and safe fallback to "off".
• The default behavior remains unchanged, and performance is preserved in the "off" fast path.

hi @ben-schwen , when you have time could you please take a look?
thanks.

[github-actions]

No obvious timing issues in HEAD=issuenight

Generated via commit 88085d1

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

Task	Duration
R setup and installing dependencies	5 minutes and 44 seconds
Installing different package versions	11 minutes and 46 seconds
Running and plotting the test cases	4 minutes and 18 seconds