GH-51311: [C++][Compute] Add float16 to FloatingPointTypes

[Reranko05]

Rationale

FloatingPointTypes() currently contains only float32() and float64(), so float16() is also omitted from NumericTypes() and PrimitiveTypes().

This was noted as a follow-up during the review of #50513.

Changes

• Add float16() to FloatingPointTypes().
• NumericTypes() and PrimitiveTypes() automatically include float16() through their existing composition from FloatingPointTypes().

Testing

• arrow-type-test passes.

GitHub Issue: #51311

• GitHub Issue: [C++] Make float16 part of FloatingPointTypes() #51311

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[github-actions]

⚠️ GitHub issue #51311 has been automatically assigned in GitHub to PR creator.

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[Reranko05]

Once #50513 is merged, we can verify that the updated type list works correctly.

[pitrou]

@Reranko05 , as I expected, this is breaking a lot of existing tests for functionality that doesn't accept Float16 currently.

The easy way out is to avoid running those tests on Float16, but it would be nicer of course to progressively have all the tested functionality accept Float16.

[Reranko05]

@pitrou Okay, so should I update the affected tests to avoid float16 for now?

[pitrou]

Yes, probably. Perhaps using a test-only function that would return floating-point types without Float16.

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[Reranko05]

@pitrou NumericTypes() gets float16 indirectly through g_floating_types. Once float16 is included, production registration paths iterating over NumericTypes() can pass Type::HALF_FLOAT to ArithmeticExecFromOp(), which currently only has cases for FLOAT and DOUBLE (float32/float64). So this needs to be handled separately from the test-only exclusions.