Fix fuzzy set union boundaries

[AbodiDawoud]

Describe your change:

Fixes #11871.

This fixes FuzzySet.union() so the returned triangular fuzzy set keeps its fields in boundary order:

• left_boundary: minimum left boundary
• peak: maximum peak
• right_boundary: maximum right boundary

Previously the method placed the maximum right boundary in peak and the average peak in right_boundary, which produced invalid output such as peak=0.6, right_boundary=0.35.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

Local checks:

• .venv/bin/python -m doctest -v fuzzy_logic/fuzzy_operations.py
• .venv/bin/python -m pytest fuzzy_logic/fuzzy_operations.py
• .venv/bin/ruff check fuzzy_logic/fuzzy_operations.py
• .venv/bin/ruff format --check fuzzy_logic/fuzzy_operations.py
• .venv/bin/mypy --ignore-missing-imports fuzzy_logic/fuzzy_operations.py
• git diff --check

[diusazzad]

Could you please add a small note in the PR description or docstring explaining the rationale behind using the average for the peak and right_boundary? It would be great to know if this follows a specific fuzzy logic convention.