Use build-local dir instead of project deps in samples

Our Spring Boot + OTel samples currently use project dependencies (e.g. `project(":sentry-opentelemetry:sentry-opentelemetry-bom")`), which:

- Pollutes the buildscript classpath, undermining test isolation
- Makes the samples uncopyable by customers (who won't have project references)
- Masks real compatibility issues (e.g. OTel + Spring Boot version conflicts hidden by the BOM being pulled via project ref rather than a real artifact)

**Proposed change**

Publish SDK artifacts to a build-local Maven directory during CI and configure the samples to resolve from that directory. This preserves per-PR test coverage without the classpath isolation problems of `mavenLocal` (which has shared global state) and without requiring publishing to Maven Central.

**Scope**

- Set up a build-local publish step that outputs to a local directory (e.g. `build/local-repo`)
- Update sample `build.gradle.kts` files to add that local dir as a repository
- Use published coordinates (e.g. `io.sentry:sentry-opentelemetry-bom`) instead of project references in `dependencyManagement`
- Keep samples using released/stable version strings so customers can copy them as-is

**Out of scope**

- Moving samples to Maven Central publishing
- Fully separating samples from test infrastructure (tracked separately)

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[View Junior Session in Sentry](https://sentry.sentry.io/explore/conversations/slack%3AC089VFRB8N9%3A1783054068.421249/?project=4510944073809921)

Comece lendo os arquivos build.gradle.kts de exemplo e a configuração de build do CI que publica artefatos do SDK. Rastreie a configuração atual das dependências do projeto e de dependencyManagement e, em seguida, verifique se os exemplos resolvem as coordenadas publicadas a partir do diretório build-local com strings de versão estáveis e se os testes de exemplo do CI passam.

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