ENH #61033: Add coalesce_keys option to DataFrame.join for preserving join keys
Add coalesce_keys option to DataFrame.join for preserving join keys

This adds a coalesce_keys keyword to DataFrame.join to allow preservation
of both join key columns (id and id_right), instead of automatically
coalescing them into a single column.

This is especially useful in full outer joins, where retaining information
about unmatched keys from both sides is important.

Example:
    df1.join(df2, on=id, coalesce_keys=False)

This will result in both id and id_right columns being preserved, rather
than merged into a single id.

Includes:
- Modifications to join internals (core/reshape/merge.py)
- A dedicated test file (test_merge_coalesce.py) covering:
    - Preservation of join keys when coalesce_keys=False
    - Comparison with default behavior (coalesce_keys=True)
    - Full outer joins with asymmetric key presence

closes #61033 This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.