RFR: 8371091: Improve the exception message of NullPointerException thrown by the methods in the default implementation of HttpRequest.Builder [v2]

[EunHyunsu]

> This change adds clearer NullPointerException messages to the POST(), PUT(), and method() methods in HttpRequest.Builder when the provided BodyPublisher is null.
> 
> Each of these methods now calls `requireNonNull(body, "BodyPublisher must not be null")`, making the exception message explicit.
> A corresponding regression test (RequestBuilderNullBodyTest) has been added to verify this behavior.
> 
> All builds and related tests (jdk/java/net/httpclient) have passed successfully.
> 
> Currently, only POST(), PUT(), and method() include these messages.
> Some other builder methods (such as header(), setHeader()) also throw NullPointerException without a message.
> I would like to discuss whether adding consistent messages to those methods would be beneficial,
> and whether the current wording "BodyPublisher must not be null" feels appropriate.

EunHyunsu has updated the pull request incrementally with one additional commit since the last revision:

  8271091: Add NPE messages for method

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/28103/files
  - new: https://git.openjdk.org/jdk/pull/28103/files/169294d3..24ecfc3b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28103&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28103&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28103.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28103/head:pull/28103

PR: https://git.openjdk.org/jdk/pull/28103