Port MessageFactory-namespaced logger registry from 2.x

[vpelikh]

Summary

Completes the port of #2962 by removing the now-dead checkMessageFactory machinery from LoggerContext.
The MessageFactory-namespaced InternalLoggerRegistry itself was already ported in #4157. This PR removes the leftover core code that only existed to warn about message-factory mismatches under the old name-keyed registry.

Why checkMessageFactory was dead code

The LoggerRegistry now keys loggers by (name, message factory). As a result:

• computeIfAbsent(name, mf, newLogger) always returns a logger whose message factory matches the requested one, so the warning branch !loggerMessageFactory.equals(messageFactory) could never fire.
• getLogger normalizes a null message factory to the default before reaching the registry, so the null-mismatch branch was unreachable as well.

The behavior this method guarded against — returning an existing logger created with a different message factory — no longer exists: a request with a different factory now returns a distinct, correctly-bound logger instead of the old logger plus a warning. 2.x removed this dead method in PR #2936, the same change that introduced the namespaced registry there; this PR mirrors that change in the 3.x, keeping it a faithful port.

Note on registry placement: as suggested in #2962 (comment), the namespaced registry was kept out of log4j-api; it lives in log4j-core (mirroring 2.x) rather than log4j-kit, since kit sits below core and can't reference the core Logger type.

Changes

1. log4j-core — LoggerContext
   • Removed private checkMessageFactory(ExtendedLogger, MessageFactory) and its call site in getLogger(String, MessageFactory).
   • Removed the now-unused ExtendedLogger and StatusLogger imports.
   • Added @Nullable to the message-factory parameter of getLogger(String, MessageFactory) and hasLogger(String, MessageFactory); refreshed the getLogger Javadoc to describe the message-factory-matching semantics.
2. log4j-core-test — LoggerTest
   • Added regression test getLogger_String_MessageFactoryMismatchProducesNoWarning: re-requests a logger with a different and with a null message factory, and asserts no "created with the message factory" status warning is emitted.
   • Kept the builder hardcoded source-line constant in sync with the file layout.

Closes #2962