remotewrite/sender: Migrate Remote Write 2.0 sender tests to new framework

[bwplotka]

This PR finishes migrating the Remote Write 2.0 sender compliance tests to the new test framework defined in compliance.go, validating against the Prometheus Remote-Write 2.0 Specification.

Stacked on top of #256.

Summary of Changes

• Migrated 15 sender test files from the deprecated structure to the slice-based Test framework:
  • backoff_test.go
  • batching_test.go
  • combined_test.go
  • edge_cases_test.go
  • error_handling_test.go
  • exemplars_test.go
  • fallback_test.go
  • histograms_test.go
  • labels_test.go
  • metadata_test.go
  • protocol_test.go
  • response_test.go
  • retry_test.go
  • rw1_compat_test.go
  • symbols_test.go
• Updated ComplianceTests() in compliance.go to register all test suites.
• Cleaned up rules that did not align with the PRW 2.0 specification (e.g. arbitrary batch size limits, strict exponential backoff requirements, heuristic symbol reuse tests).
• Verified tests pass with go test ./sender -v.