fix: decouple pmd, spotbugs, and checkstyle skip flags from skiptests

[aws-kevinrickard]

Problem

maven-pmd-plugin, spotbugs-maven-plugin, and maven-checkstyle-plugin all had their <configuration><skip> wired to the single ${skipTests} property instead of their own idiomatic skip parameters. Concretely, in pom.xml:

<!-- maven-pmd-plugin -->
<skip>${skipTests}</skip>

<!-- spotbugs-maven-plugin -->
<skip>${skipTests}</skip>

<!-- maven-checkstyle-plugin -->
<skip>${skipTests}</skip>

This causes two problems:

1. Each plugin's own documented skip flag silently does nothing. For example, -Dspotbugs.skip=true is the SpotBugs Maven plugin's own parameter, but since the POM never reads spotbugs.skip, passing it has zero effect — the analysis still runs. Same for -Dpmd.skip=true and -Dcheckstyle.skip=true.
2. -DskipTests=true conflates two different intents: "skip running the test suite" and "skip static analysis." Passing it skips everything (tests + PMD + SpotBugs + Checkstyle), which is surprising if you only wanted to skip one of those.

Fix

Each plugin now reads its own skip property, added to <properties> with the same default (false) skipTests already had:

<pmd.skip>false</pmd.skip>
<spotbugs.skip>false</spotbugs.skip>
<checkstyle.skip>false</checkstyle.skip>

And each plugin's <configuration><skip> now points at its own property instead of ${skipTests}.

skipTests is untouched everywhere else (surefire, JaCoCo) — it continues to control only test execution, as before.

Behavior after this change

• Default mvn test (no extra flags): identical to before — PMD, SpotBugs, Checkstyle, and the test suite all run, since the new properties default to false just like skipTests did.
• -Dpmd.skip=true: now actually skips PMD, while Checkstyle/SpotBugs/tests still run.
• -Dcheckstyle.skip=true: now actually skips Checkstyle, while PMD/SpotBugs/tests still run.
• -Dspotbugs.skip=true: now actually skips SpotBugs, while PMD/Checkstyle/tests still run.
• -DskipTests=true: still skips test execution (unchanged), but no longer skips PMD/Checkstyle/SpotBugs — each of those needs its own explicit flag now. This is the corrected, intended behavior: previously -DskipTests=true skipped everything, which was itself the confusing overload this PR removes.

Testing

Verified locally via full mvn test lifecycle runs:

• Default run (no flags): PMD and Checkstyle execute and pass, matching pre-change behavior.
• -Dpmd.skip=true: PMD produces no report / does not run (confirmed via missing target/pmd.xml and near-instant mojo execution time vs. the multi-second scan when enabled); Checkstyle still runs.
• -Dcheckstyle.skip=true: Checkstyle mojo execution drops from ~4s to ~1s (confirming it no-ops) while PMD still runs and produces its report.
• -DskipTests=true alone: test execution is skipped (Tests are skipped.), while PMD and SpotBugs still execute in the test phase as before.
• Confirmed no regression to default behavior and no changes to the spotbugs-maven-plugin <version>, JaCoCo, or surefire configuration.

Note: this environment's JDK (11/17/21) triggers a known, pre-existing, unrelated SpotBugs 4.0.0 crash (Unsupported class file major version 61) when actually running the SpotBugs analysis — reproduced identically on unmodified main before this change, so it isn't something introduced here. -Dspotbugs.skip=true was used to work around it when verifying the other flags, and is itself one of the flags this PR fixes.