Detect file collisions in SiftingAppender nested appenders (#1041)

[seonwooj0810]

Fixes #1041

Root cause

FileCollisionAnalyser runs during the DEPENDENCY_ANALYSIS phase and only inspects statically declared FileAppender/RollingFileAppender models. A SiftingAppender instantiates its nested appender once per discriminator value at runtime, so the nested appender never passes through the static collision maps. When the nested file/fileNamePattern does not embed the discriminator key, every sifted instance resolves to the same target file and their output silently collides — exactly the case @ceki confirmed and scoped in the issue ("add an analyzer for the case the appender inside SiftingAppender does not make use of the discriminating key").

Change

When an AppenderModel contains a SiftModel, the analyser now:

1. resolves the discriminator key from the model tree (<discriminator><Key>…</Key>),
2. reads the nested appender's file/fileNamePattern raw body text (raw on purpose — the key is only bound as a substitution property at runtime), and
3. if none of them reference ${<key>}, emits a warning naming the appender, the discriminator key, and the shared target file.

The check stays silent when the key cannot be determined from the configuration (e.g. a discriminator using a built-in default key) or when the nested appender writes to no file (e.g. a ConsoleAppender), to avoid false positives. It is a warning rather than a hard error/skip because the colliding appenders only exist at runtime and cannot be skipped at model-analysis time.

Change summary

• FileCollisionAnalyser: detect the SiftingAppender variant and warn (SIFT_COLLISION_MESSAGE).
• New tests + input configs.

Test evidence

Added to FileCollisionAnalyserTest:

• siftingAppenderSharedFileIsDetected — nested <file> does not reference the discriminator key → warning is emitted (siftShared.xml, mirrors the issue's reproduction).
• siftingAppenderDistinctFileIsNotFlagged — nested <file> embeds ${userId} → no warning (siftDistinct.xml).

Verification done: built logback-core, ran FileCollisionAnalyserTest (7/7 pass, incl. the 5 pre-existing collision tests) and the full *Sift* suite (14 run, 0 failures) — no regressions. JDK 21.

Automated tooling was used to help prepare this change; it has been reviewed and verified by a human.