Add BytesView constructors and unchecked string conversion

[bobzhang]

Motivation

• Align BytesView API with Bytes by providing constructors and the unchecked string helper that mirror Bytes counterparts.
• Expose the new BytesView helpers in the builtins interface so external code can use them without workarounds.

Description

• Added BytesView::from_array(ArrayView[Byte]) -> BytesView to construct a view from an array by delegating to Bytes::from_array(... )[:].
• Added BytesView::from_iter(Iter[Byte]) -> BytesView (declared with #alias(from_iterator, deprecated)) to construct a view from an iterator by delegating to Bytes::from_iter(... )[:].
• Added BytesView::to_unchecked_string(self, offset? : Int, length? : Int) -> String which delegates to Bytes::to_unchecked_string using the view start offset and validates bounds.
• Regenerated the builtin package interface (pkg.generated.mbti) so the new BytesView APIs are published.

Testing

• Ran moon info which completed successfully and updated the generated interface without errors.
• Ran moon fmt which completed successfully and formatted the modified files.

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Codex Task

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 3 additional flags.

[coveralls]

Pull Request Test Coverage Report for Build 2526

Details

• 0 of 5 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.04%) to 95.952%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
builtin/bytesview.mbt	0	5	0.0%

Totals
Change from base Build 2524:	-0.04%
Covered Lines:	11448
Relevant Lines:	11931

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💛 - Coveralls