Migrate TestValidateIntegrationPolicyTemplates_DataStreamStreams complex cases to test/packages/ fixtures

[coderabbitai]

Summary

The complex multi-file package layout test cases inside TestValidateIntegrationPolicyTemplates_DataStreamStreams in code/go/internal/validator/semantic/validate_integration_policy_template_path_test.go should be migrated to fixture-based validator tests under test/packages/, exercised via code/go/pkg/validator/validator_test.go.

Background

Per the repository's semantic-validator testing contract (see AGENTS.md): for complex scenarios with multiple files and directories, use test packages in test/packages/ tested via validator_test.go. Simple unit scenarios (single file or small structure) are appropriate for t.TempDir() unit tests.

The cases that build multi-file layouts (root manifest, agent/input, data_stream/*/manifest.yml, agent/stream templates) should be extracted into new package fixtures under test/packages/ with corresponding entries in validator_test.go asserting the same expected errors/results.

Context

• Identified during review of PR Align integration template path validation with Fleet #1122 (Align integration template path validation with Fleet #1122)
• Review comment: Align integration template path validation with Fleet #1122 (comment)
• Requested by: @teresaromero

Acceptance Criteria

• Extract integration-level cases from TestValidateIntegrationPolicyTemplates_DataStreamStreams into new test/packages/ fixtures
• Add corresponding test assertions in validator_test.go
• Keep only simple/single-file unit scenarios in the semantic package test file