Separate sample projects from test infrastructure (converted to project)

This issue has been converted to a project [Separate sample projects from test infrastructure]()

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Currently our samples double as integration tests, which causes buildscript classpath bleed between the main build and the sample projects. This creates two categories of problems:

**Isolation failures observed**

* A test matrix leg targeting a specific KGP version was silently resolved to a higher version because another build dependency pulled it in transitively
* Sample dependency constraints (e.g. an older Spring Boot requiring a pinned KGP version) block upgrades to the rest of the build

**Goals**

* Samples should use only published Maven Central coordinates so customers can copy them verbatim — no project references, no snapshots
* Integration tests should live in a separate Gradle project (or composite build) with an isolated buildscript classpath, publishing to a build-local directory for resolution
* Changes to the main build graph should not be able to silently alter what the samples or integration tests resolve

**Proposed direction**

* Extract sample apps into standalone Gradle projects (or an isolated composite build) that declare no `project(...)` dependencies
* Run integration/system tests against those standalone projects, resolving SDK artifacts from a build-local Maven directory (see [JAVA-625]())
* CI wires the publish-to-local-dir step before the sample test step, keeping full per-PR coverage

**Out of scope**

* The build-local dir plumbing itself (tracked in [JAVA-625]())
* Publishing samples to any external registry

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[View Junior Session in Sentry]()

Work is tracked in the linked Linear project rather than this issue. Start by mapping the current sample Gradle projects, integration/system tests, and CI wiring, then review JAVA-625 for the build-local resolution dependency; done means samples use published coordinates while isolated integration tests retain full per-PR coverage.

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