[Hanabi9248]

Two public StringUtil API paths return incorrect results for supported inputs:

• Reverse case-insensitive search skips index zero, so an existing prefix match returns -1. Include index zero while retaining the original starting-index normalization.
• Case-sensitive equals accepts CharSequence but calls object equality. Distinct StringBuilder/StringBuffer objects containing the same text, or mixed String/builder operands, therefore compare unequal. Compare both textual contents as the existing ignore-case branch already does; preserve null guards and case sensitivity.

The fixes are separate commits and regression runners. The equality addition is based on the already published reverse fix; it retains that production hunk byte-for-byte.

Validation:

• Reverse fix, earlier accepted version 61dbe1b: baseline four assertion failures and four controls, candidate eight pass. This unchanged regression was not rerun for the equality addition.
• Equality addition at c8da512 (production blob 0bcc35d121c9dcdbc11b27d1b92f7d148cf0e85f): complete StringUtil plus four unchanged official support classes and real Spring Core5.2.10.RELEASE. Baseline six assertion failures and eight controls; candidate fourteen pass. Assertions cover builders, buffers, mixed String/CharSequence and both overloads; controls retain unequal/case/identity/null behavior.
• JDK25 javac --release8 and runtime25, CPU1/aggregate768MiB/16processes/privateTMP/BLAS1. Equality job peak313683968 bytes. No installed dependencies, stubs or copied function bodies. --release8 is not Java8 runtime testing.
• Full Maven/Spring service, Java8 runtime and whole repository NOT_RUN. Earlier eight-pass evidence and new fourteen-pass evidence are separate runs, not a new combined22 pass claim. No affected controller endpoint is asserted; this is the documented public utility API.

Run focused equality test: compile the complete class, its four official support classes and tests/StringUtilCharSequenceEqualsRegression.java against the existing Spring Core5.2.10.RELEASE, then run that class. Existing tests/StringUtilReverseSearchRegression.java remains unchanged.

Assisted-by: OpenAI gpt-6.1-sol