[main] Add @Required and validation to StringMatchFilter text attribute (#3158 port)

[vpelikh]

Ports the StringMatchFilter validation work from the 2.x branch (#3158), originally started by @JWT007 in #3510 (his commit authorship is preserved), and resolves the earlier review feedback.

Changes

• StringMatchFilter.java: The text builder attribute is now marked @Required(message = "No text provided for StringMatchFilter") so a missing attribute fails configuration validation. The constructor validates with Assert.requireNonEmpty(...). The builder's setText(@NonNull String) throws NullPointerException for a null argument and IllegalArgumentException for an empty string. build() returns null (logged error) when text is missing, and a new getText() accessor was added.
• StringMatchFilterTest.java: Added tests covering valid construction, empty text rejection, null text rejection (NullPointerException), missing-attribute rejection via config, and the post-stop() guard.
• Test configs: Added log4j2-stringmatchfilter-3153-ok.xml and log4j2-stringmatchfilter-3153-nok.xml fixtures. The nok config uses a correctly named <StringMatchFilter/> element with no text attribute (the earlier version used a misspelled plugin name and was replaced).

Why

An empty or missing text attribute on StringMatchFilter previously resulted in either a cryptic NullPointerException at runtime (after stop()) or silently matching the empty string against every event. This change provides clear validation at configuration time and graceful degradation.

Notes

• Out-of-scope refactors to AbstractFilter/AbstractLifeCycle/Assert (equalsImpl, etc.) that appeared in the original [main] Add @Required and validation to StringMatchFilter text attribute (#3158 port) #4153 diff were dropped to keep this PR focused on StringMatchFilter validation; they can be handled in a follow-up.

[ramanathan1504]

@vpelikh

It turns out the original author (@JWT007) already started porting this to main over in PR #3510, but that PR stalled and still has some unresolved review comments from @vy and @ppkarwasz .

Since he already did half the work, could you pull his commits into this PR (to preserve his authorship), and then address the review feedback that @vy and @ppkarwasz left on #3510?

If you can resolve those review comments here in this PR, I will close the old PR and we can focus on getting yours reviewed and merged!

[vpelikh]

Thanks @ramanathan1504 — I've pulled @JWT007's original port commits from #3510 into this PR (authorship preserved) and addressed the open review feedback that @vy and @ppkarwasz left there:

@ppkarwasz — setText(null) should throw NPE (not IllegalArgumentException)

• setText(@NonNull final String text) now calls Objects.requireNonNull(text, ...) first, so a null argument throws NullPointerException (the @NonNull contract violation).
• An empty string still throws IllegalArgumentException via Assert.requireNonEmpty, keeping the two validation levels distinct (null → NPE, empty → IAE). The corresponding test now asserts NullPointerException.

@ppkarwasz — the misspelled-plugin-name test didn't belong here

• The nok fixture (StringMatchFfilter, a typo) is replaced with a correct <StringMatchFilter/> element that omits the required text attribute, so the test now exercises StringMatchFilter's own @Required validation (the plugin is skipped → getFilter() is null) rather than PluginBuilder name resolution.

@vy / @ppkarwasz — avoid cosmetic changes on existing code

• Reverted the method reordering and the rewritten filter() javadocs back to the original main form; only the validation behavior was added. The filter() method bodies are unchanged.
• Dropped the out-of-scope equalsImpl refactors in AbstractFilter/AbstractLifeCycle and the Assert tweaks into a separate follow-up change, keeping this PR focused on StringMatchFilter validation.