Add opt-out for default RestClient status handler

[seungy0]

Summary

Introduce an explicit opt-out for the built-in default RestClient status handler (4xx/5xx → exception).
This allows users to fully control status handling without the “always-match” workaround.

Motivation

• Default StatusHandler is added implicitly, which is hard to discover.
• NoOpResponseErrorHandler does not disable the built-in handler.
• Treating some 4xx/5xx as normal outcomes is currently cumbersome.

Related issue: #36248

Changes

• Add RestClient.Builder#disableDefaultStatusHandler() (default behavior unchanged).
• Skip built-in handler registration when disabled.
• Update Javadoc to document the opt-out.
• Tests verifying default behavior vs opt-out.

Tests

• DefaultRestClientTests.defaultStatusHandlerThrowsOnErrorStatus
• DefaultRestClientTests.disableDefaultStatusHandlerAllowsErrorBody
• RestClientBuilderTests.disableDefaultStatusHandler

Notes

Backward compatible. Opt-in only.

[LavaC3]

@seungy0
Thanks for your feedback and fixing this issue perfectly!

🙏 I realize my previous implementation had an oversight (didn't consider the built-in StatusHandler logic in DefaultResponseSpec), and your solution with a dedicated disable flag passed to DefaultResponseSpec is definitely the right and complete approach.

It's a great learning for me to dive into the RestClient source code and Spring contribution process through this issue.