ty: un-ignore unsupported-operator rule

[SirHumza]

Describe your change

• 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?

Contributes to #15187.

Removes rules.unsupported-operator = "ignore" from [tool.ty]. On a synced Python 3.15 (regular GIL) environment the rule now reports zero diagnostics, so it can be enforced without any new findings.

Files fixed:

• sorts/iterative_merge_sort.py, sorts/quick_sort_3_partition.py, sorts/recursive_quick_sort.py -- the local Comparable protocols only declared the dunder used in one spot, so the operators at the call sites were flagged as unsupported. Declared the missing dunders (__le__, __ge__, __gt__ respectively), matching the pattern in sorts/merge_sort.py / sorts/quick_sort.py.
• computer_vision/mean_threshold.py -- Image.load() is PixelAccess | None and grayscale pixels are read as float | tuple[int, ...], so the average/compare-and-write loops tripped unsupported-operator (plus an invalid-assignment on the None subscript). Added a None guard and narrowed the pixel to a numeric value, scoped to the grayscale images the docstring already requires.
• web_programming/get_amazon_product_data.py -- build the product link with an f-string instead of "prefix" + item.h2.a["href"], since the attribute value is not statically str. Matches the f-string already used at line 28.

Verification (uv sync --python 3.15, then uv run --python 3.15 --with ty ty check):

• No unsupported-operator diagnostics remain and the total diagnostic count does not increase.
• doctests: sorts/iterative_merge_sort.py 16 passed, sorts/quick_sort_3_partition.py 39 passed, sorts/recursive_quick_sort.py 9 passed, computer_vision/mean_threshold.py passed.
• ruff check and ruff format --check clean on the touched files.

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".