Adjust StringBuilder size hint semantics

[bobzhang]

Summary

• change StringBuilder() to use an eager default capacity hint of 8 UTF-16 code units
• define size_hint consistently as UTF-16 code-unit capacity rather than bytes
• keep StringBuilder(size_hint=0) as the explicit empty-buffer path and make buffer growth handle zero capacity
• reject negative size_hint values on both buffer-backed and JS concat-backed implementations
• add tests for explicit zero-capacity growth and negative hint rejection

Validation

• moon fmt builtin/stringbuilder_buffer.mbt builtin/stringbuilder_concat.mbt builtin/stringbuilder_test.mbt
• moon check builtin
• moon test builtin
• moon info
• moon ide doc StringBuilder::StringBuilder
• git diff --check
• moon check
• moon test

[coveralls]

Coverage Report for CI Build 4484

Coverage remained the same at 94.089%

Details

• Coverage remained the same as the base build.
• Patch coverage: 3 of 3 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	15853
Covered Lines:	14916
Line Coverage:	94.09%
Coverage Strength:	198865.29 hits per line

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💛 - Coveralls

[Copilot]

Pull request overview

Updates StringBuilder constructor semantics to treat size_hint consistently as a UTF-16 code-unit capacity hint, with a small eager default to reduce early reallocations, and aligns behavior across the buffer-backed (non-JS) and concat-backed (JS) implementations.

Changes:

• Changed StringBuilder() default size_hint from 0 to 8 (UTF-16 code units) and updated docstrings accordingly.
• Made size_hint=0 a supported “empty initial buffer” path by allowing zero-capacity buffers and updating growth logic to handle it.
• Rejected negative size_hint values in both implementations and added tests for zero-capacity growth and negative-hint rejection.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
builtin/stringbuilder_buffer.mbt	Switches to UTF-16 code-unit capacity hints, allows 0-capacity buffers, and updates growth logic to handle zero capacity.
builtin/stringbuilder_concat.mbt	Aligns constructor defaults/docs and rejects negative size_hint even though the hint is ignored on JS.
builtin/stringbuilder_test.mbt	Adds coverage for size_hint=0 growth and negative size_hint constructor panics.

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