Add Gradle contextual analysis support

[omribz156]

Summary:

• include Gradle in the repository-config package types that keep vuln_contextual_analysis in API payloads and state
• update the Gradle acceptance-test fixture to exercise vuln_contextual_analysis
• update the checked-in repository config docs to list Gradle support

This follows the existing Gradle JFrog Advanced Security support gate used for exposures/secrets scans.

Verification:

• gofmt -w pkg/xray/resource/resource_xray_repository_config.go pkg/xray/resource/resource_xray_repository_config_test.go
• git diff --check
• go test ./pkg/xray/resource -run TestDoesNotExist -count=0
• go test ./pkg/xray/resource -run TestAccRepositoryConfig_RepoConfig_Create_VulnContextualAnalysis -count=1 -v (skips without JFROG_JAS_DISABLED=false, as expected for this acceptance test)

Note: I also tried go generate ./..., but it could not complete because tfplugindocs was unable to download Terraform after checksum signature verification failed with openpgp: key expired.

Fixes #416.

Implemented with Codex assistance; I reviewed the issue, source references, and final diff manually before opening this PR.