Add compatibility matrix tests for multiple SpiceDB versions and TLS modes

[rawkode]

Problem

Integration tests currently pin a single SpiceDB version (v1.45.4 in tests/integration.rs) and do not cover TLS/transport matrix combinations.

Impact

Upgrade regressions and environment-specific issues may only be discovered in production.

Proposed change

• Add CI matrix coverage for at least:
  • latest SpiceDB version,
  • previous minor SpiceDB version,
  • plaintext mode,
  • TLS mode.
• Ensure integration tests run across all matrix entries.
• Keep matrix configuration maintainable and documented.

Acceptance criteria

• CI runs integration tests across the defined SpiceDB version + TLS matrix.
• Failures are attributable to matrix dimensions in CI output.
• Baseline matrix includes latest + previous minor and plaintext + TLS.
• Documentation explains how to run/select matrix variants locally.