fix: scope logging handler to haystack logger, not root

[sanjuchand]

Summary

Fixes #8681.

configure_logging() was adding its StreamHandler to the root logger, which violates Python's logging best practices for libraries and silently breaks any application that calls logging.basicConfig() after importing Haystack — basicConfig is documented as a no-op once any handlers are already registered on the root logger.

Change: Scope the HaystackLoggingHandler to logging.getLogger("haystack") instead of logging.getLogger() (root). This means the structured formatter applies to all haystack.* loggers as before, but no longer pollutes the root logger.

Test plan

• Updated reset_logging_config fixture to reset haystack logger handlers (not root)
• Updated all test loggers to use haystack.test namespace so they route through the scoped handler
• Added TestRootLoggerNotModified with two new tests:
  • test_configure_logging_does_not_add_handler_to_root_logger — regression test for the exact bug
  • test_configure_logging_adds_handler_to_haystack_logger — confirms the handler lands in the right place
• Release note included

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[sanjuchand]

Closing as a duplicate of #11553, which covers the same fix and was opened earlier today. Good luck with the merge!