RFR: 8366421: ModifiedUtf.utfLen may overflow for giant string [v4]

[Guanqiang Han]

> Please review this patch.
> 
> **Description:**
> 
> Currently, ModifiedUtf.utfLen returns a signed int. For very large strings, this may overflow and produce negative values, leading to incorrect behavior in code that relies on the UTF length. This patch changes the return type to long, which fully resolves the issue and allows safe handling of giant strings.
> 
> **Test:**
> 
> GHA

Guanqiang Han has updated the pull request incrementally with two additional commits since the last revision:

 - Update TestUtfLen.java
   
   fix format error
 - Update TestUtfLen.java
   
   Optimize regression test

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/27285/files
  - new: https://git.openjdk.org/jdk/pull/27285/files/e6122ec8..51d57c60

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27285&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27285&range=02-03

  Stats: 60 lines in 1 file changed: 50 ins; 6 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/27285.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27285/head:pull/27285

PR: https://git.openjdk.org/jdk/pull/27285