String utilities perf improvements and rtrim bug fix

[dmitrivMS]

Small perf improvements + fixed a bug in rtrim where it didn't handle multi-char suffixes correctly.
Perf measurement results from different models are below.

Codex

Scenario	Old (ms)	New (ms)	Δ vs old
ltrim · leading-spaces	1331.26	295.22	+350.94%
ltrim · leading-comment-markers	197.20	82.54	+138.92%
ltrim · leading-no-match	18.17	10.86	+67.33%
rtrim · trailing-spaces	4184.59	454.51	+820.67%
rtrim · trailing-comment-markers	169.63	66.30	+155.85%
rtrim · trailing-no-match	21.58	11.21	+92.44%

Gemini

ltrim

Scenario	Old (ms)	New (ms)	Improvement
Short string, 1 char needle	8.26	4.83	41.56%
Short string, long needle	6.30	5.99	5.00%
Long string, 1 char needle	955.75	322.78	66.23%
Long string, long needle	2582.88	1490.81	42.28%
No match	3.55	1.81	49.07%
Full match	2555.75	1616.69	36.74%
Match at start/end only	4.93	1.76	64.29%
Pathological 1 char	9614.58	3227.39	66.43%

rtrim

Scenario	Old (ms)	New (ms)	Improvement
Short string, 1 char needle	13.74	5.27	61.63%
Short string, long needle	5.90	6.79	-14.99%
Long string, 1 char needle	2677.69	296.25	88.94%
Long string, long needle	2808.86	1563.51	44.34%
No match	2.99	1.18	60.58%
Full match	2863.60	1625.15	43.25%
Match at start/end only	5.75	1.86	67.65%
Pathological 1 char	28616.35	2964.60	89.64%

Sonet

ltrim Performance

Scenario	Old (ms)	New (ms)	Improvement	Speedup
Single char space (short string)	7.84	5.68	27.5%	1.38x
Single char space (long string)	7.18	2.70	62.4%	2.66x
Single char space (many spaces)	870.62	186.27	78.6%	4.67x
Multi-char (single occurrence)	6.21	2.92	53.0%	2.13x
No match	1.92	1.16	39.4%	1.66x
Entire string (single char)	3.36	1.52	54.9%	2.21x
Slash (single char)	5.98	2.73	54.4%	2.19x
Tab character	5.05	2.32	54.1%	2.18x
Newline character	4.68	2.21	52.8%	2.12x

ltrim Average Improvement: 53.0%

rtrim Performance

Scenario	Old (ms)	New (ms)	Improvement	Speedup
Single char space (short string)	12.42	4.64	62.6%	2.68x
Single char space (long string)	22.42	2.84	87.3%	7.89x
Single char space (many spaces)	2692.02	178.77	93.4%	15.06x
Multi-char (single occurrence)	7.31	4.12	43.7%	1.77x
No match	3.22	1.11	65.6%	2.90x
Entire string (single char)	8.29	1.52	81.7%	5.45x
Slash (single char)	14.26	2.87	79.9%	4.97x
Tab character	11.82	2.43	79.5%	4.86x
Newline character	11.61	2.15	81.4%	5.40x

rtrim Average Improvement: 75.0%

Key Insights

• rtrim shows significantly better improvements (75.0% avg) compared to ltrim (53.0% avg)
• Single-character optimizations are highly effective, especially for rtrim with many characters to trim
• Biggest win: rtrim with 1000 spaces is 15x faster (93.4% improvement)
• Multi-char scenarios still show good improvements (44-53%) despite being more complex
• Even no-match scenarios benefit from the optimization (39-66% faster)

[Copilot]

Pull Request Overview

This PR improves the performance of the ltrim and rtrim string utility functions, fixes a bug in rtrim where it didn't correctly handle multi-character suffixes, and optimizes the AmbiguousCharacters cache implementation.

Key changes:

• Optimized ltrim and rtrim with fast paths for single-character needles using charCodeAt instead of indexOf/lastIndexOf
• Fixed rtrim bug: replaced incorrect lastIndexOf logic with endsWith for proper multi-char suffix handling
• Simplified AmbiguousCharacters cache by using string concatenation as cache key instead of JSON.stringify on arrays

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/vs/base/common/strings.ts	Added single-char optimizations to ltrim/rtrim, fixed rtrim multi-char bug, simplified AmbiguousCharacters cache key
src/vs/base/test/common/strings.test.ts	Added test cases for multi-char trim scenarios to verify bug fix and prevent regressions