Summary

• Add langfuse_organization data source, the first data source in the provider, allowing users to look up an existing organization by either id or name
• Implement ValidateConfig to enforce exactly one of id or name is provided at plan time
• Add comprehensive unit tests covering schema validation, config validation (both/neither/each), read by ID, read by name, not-found, empty metadata, and API error cases

Test plan

• Unit tests pass (go test ./internal/provider/ -run TestOrganizationDataSource — 11 sub-tests)
• Manual terraform plan against local Langfuse instance: lookup by ID resolves name, lookup by name resolves ID
• Validation rejects config when both id and name are specified
• Run full test suite (make test)
• Run acceptance tests against Docker environment (make testacc)