feat(integration): add real Spring Boot 3.x/4.x integration tests

[stokpop]

Fixes#1357

What

Add integration tests using real Spring Boot fat jars from cloudfoundry/java-test-applications v1.0.0 to verify end-to-end framework injection.

Three tests:

• SB3 (Boot 3.5, Java 17): java-cfenv 3.x injection, cloud profile via RuntimeLogs
• SB4 cfenv (Boot 4.1, Java 21): java-cfenv 4.x injection, cloud profile via /active-profiles, jar presence via /loaded-jars, JDBC auto-config (spring.datasource.url/username/password)
• SB4 combined: java-cfenv + container-security-provider + cf-metrics-exporter

Also includes:

• fixtures_helper_test.go: downloads jars, pre-explodes for Docker mode (CF staging does this natively)
• framework-java-cfenv.md: new VCAP_SERVICES auto-configuration section documenting java-cfenv-all service connector pipeline
• Dead URL fix in frameworks_test.go
• README update with correct 5-level -run filter patterns
• Skip jar download in cached-buildpack mode

Test assertions backed by docs:

• Cloud profile activation (java-cfenv-all CfEnvProcessor)
• spring.datasource.url = jdbc:postgresql://... (from postgres:// URI via CfDataSourceEnvironmentPostProcessor)
• spring.datasource.username / spring.datasource.password extraction
• cf-metrics-exporter staging log output
• container-security-provider env var injection

[Copilot]

Pull request overview

Adds end-to-end Spring Boot integration coverage by running real Spring Boot 3.x/4.x fixtures (downloaded from java-test-applications) and asserting buildpack framework injection behavior (notably java-cfenv, plus combined framework scenarios).

Changes:

• Add real-jar Spring Boot integration tests (SB3 + SB4) including assertions for cloud profile activation and jar/agent presence.
• Add a helper that downloads and pre-explodes Spring Boot fat jars to match CF staging behavior.
• Update docs and a previously dead fixture URL reference.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/java/containers/spring_boot.go	Clarifies isSpringBootJar fallback heuristic behavior via comments.
src/integration/spring_boot_test.go	Adds new real Spring Boot SB3/SB4 integration test contexts and assertions.
src/integration/README.md	Updates Go test invocation examples and documents -run patterns for deep spec subtests.
src/integration/init_test.go	Downloads/explodes SB3/SB4 fixtures before running the integration suite and wires paths into SpringBoot tests.
src/integration/frameworks_test.go	Updates a jar URL in the (currently skipped) Checkmarx IAST binding test.
src/integration/fixtures_helper_test.go	Introduces jar download + zip extraction utilities to produce exploded fixture directories.

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