RFR: 8360070: AccessibleText.getBeforeIndex returns null for last character

[Abhishek Kumar]

`AccessibleText.getBeforeIndex` method returns `null for last characte`r due to the **wrong boundary value condition check**.
This method returns `null` when the `passed index parameter` is equal to `text's length` which is incorrect.
`getBeforeIndex` method should return `null` only if the **passed index parameter is less than 0 and greater than the text's length**.

After modifying the condition check, expected character is returned. Test is added to verify the check,

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Commit messages:
 - TextComponent's getBeforeIndex boundary value check fix

Changes: https://git.openjdk.org/jdk/pull/25941/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25941&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8360070
  Stats: 61 lines in 2 files changed: 59 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/25941.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25941/head:pull/25941

PR: https://git.openjdk.org/jdk/pull/25941