feat: add StringView methods for character length and range checks

[bobzhang]

No description provided.

[peter-jerry-ye-code-review]

Duplicated character counting logic across multiple helper functions

Category
Maintainability
Code Snippet
Lines 32-78 in stringview.mbt: Three separate functions (count_chars_in_range, has_at_least_n_chars, char_length_equals) contain nearly identical UTF-16 surrogate pair handling logic
Recommendation
Extract the surrogate pair handling into a shared helper function that returns the next index and character increment, then use it in all three functions to eliminate duplication
Reasoning
The current approach violates DRY principle and makes maintenance harder. Any bug fixes or optimizations to surrogate pair handling would need to be applied in three places.

Inconsistent error handling between refactored and original implementations

Category
Correctness
Code Snippet
Lines 65-68 in string.mbt: Original String::char_length had explicit bounds checking with abort(), but the new StringView implementation doesn't show equivalent validation
Recommendation
Ensure StringView::make_view and related functions perform the same bounds validation as the original String::char_length implementation, or document why it's not needed
Reasoning
The original implementation had safety checks that may have been lost during refactoring, potentially introducing runtime errors or undefined behavior.

char_length_equals function has suboptimal early termination logic

Category
Performance
Code Snippet
Lines 76-78 in stringview.mbt: count == len && index == end - checks both conditions even when count != len
Recommendation
Change to index == end && count == len to short-circuit when we haven't reached the end of the string, or restructure the loop to return false immediately when count exceeds len
Reasoning
The current order evaluates both conditions unnecessarily. When count != len, we don't need to check if index == end since the result will be false regardless.

[coveralls]

Pull Request Test Coverage Report for Build 1783

Details

• 10 of 30 (33.33%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.2%) to 89.644%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
builtin/stringview.mbt	9	29	31.03%

Files with Coverage Reduction	New Missed Lines	%
string/string.mbt	2	95.1%

Totals
Change from base Build 1782:	-0.2%
Covered Lines:	9989
Relevant Lines:	11143

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💛 - Coveralls