Fix FiniteSet union handler for symbolic elements

[gschintgen]

References to other Issues or PRs

Fixes #18241.

Brief description of what is fixed or changed

Before:

In [1]: Interval(0, 1).union(FiniteSet(1, x))
Out[1]: [0, 1]

After:

In [1]: Interval(0, 1).union(FiniteSet(1, x))
Out[1]: [0, 1] ∪ {x}

Reason:
Incorrect handling of .contains() ternary logic
(True or False or Contains/Relational)

Tests are added.

Other comments

Release Notes

• sets
  • Fixed a bug in FiniteSet's union handler that could lead to some elements being "dropped" if their membership in the other set can't be determined.

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• sets
  • Fixed a bug in FiniteSet's union handler that could lead to some elements being "dropped" if their membership in the other set can't be determined. (#18356 by @gschintgen)

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#### References to other Issues or PRs

Fixes #18241.

#### Brief description of what is fixed or changed

Before:
```
In [1]: Interval(0, 1).union(FiniteSet(1, x))
Out[1]: [0, 1]
```

After:
```
In [1]: Interval(0, 1).union(FiniteSet(1, x))
Out[1]: [0, 1] ∪ {x}
```

Reason:
Incorrect handling of `.contains()` ternary logic
(True or False or Contains/Relational)

Tests are added.

#### Other comments


#### Release Notes

<!-- Write the release notes for this release below. See
https://github.com/sympy/sympy/wiki/Writing-Release-Notes for more information
on how to write release notes. The bot will check your release notes
automatically to see if they are formatted correctly. -->

<!-- BEGIN RELEASE NOTES -->
* sets
  * Fixed a bug in `FiniteSet`'s union handler that could lead to some elements being "dropped" if their membership in the other set can't be determined.
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[codecov]

Codecov Report

Merging #18356 into master will decrease coverage by 0.002%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##            master    #18356       +/-   ##
=============================================
- Coverage   75.013%   75.011%   -0.003%     
=============================================
  Files          649       649               
  Lines       167348    167348               
  Branches     39417     39417               
=============================================
- Hits        125534    125530        -4     
- Misses       36275     36284        +9     
+ Partials      5539      5534        -5

[oscarbenjamin]

Looks good.