refactor: reuse main java/proto sources from base modules in *-otel-* examples

[Copilot]

The 6 new *-otel-* example modules introduced in PR #450 duplicated their src/main sources verbatim from gcp-grpc-spring-without-test-artifacts-example and gcp-web-without-test-artifacts-example. This replaces those copies with cross-project sourceSets imports.

gRPC otel modules (×4) — full import

All four modules had sources 100% identical to gcp-grpc-spring-without-test-artifacts-example. src/main/{java,proto,resources} deleted; imported via sourceSets:

// e.g. gcp-grpc-spring-otel-agent-example/build.gradle
sourceSets {
    main {
        java     { srcDirs '../gcp-grpc-spring-without-test-artifacts-example/src/main/java' }
        proto    { srcDirs '../gcp-grpc-spring-without-test-artifacts-example/src/main/proto' }
        resources { srcDirs '../gcp-grpc-spring-without-test-artifacts-example/src/main/resources' }
    }
    test.java.srcDirs += "${protobuf.generatedFilesBaseDir}/test/java"
    test.java.srcDirs += "${protobuf.generatedFilesBaseDir}/test/grpc"
}

Applies to: gcp-grpc-spring-otel-{agent,starter}-example and gcp-grpc-spring-without-test-artifacts-otel-{agent,starter}-example.

Web without-test-artifacts otel modules (×2) — java import

• gcp-web-without-test-artifacts-otel-agent-example: imports main java + resources from base (identical after the fix below)
• gcp-web-without-test-artifacts-otel-starter-example: imports main java; keeps its own application.properties (adds management.tracing.* properties required by the OTel Spring Boot starter)

Base web module alignment

gcp-web-without-test-artifacts-example/DocumentEndpoint.java updated to log the MDC context map — a line already present in all otel variants, useful regardless of OTel. Uses SLF4J parameterised logging ({}); same fix applied to the two remaining web otel DocumentEndpoint copies that are not yet deduplicated.

Not included

gcp-web-otel-{agent,starter}-example have extra classes (RestClientConfig, UserIdEnricherFilter, DummyEndpoint, etc.) and diverged versions of WebSecurityConfig/DocumentEndpoint vs the base — a clean import would require restructuring those source trees and is deferred.