Constitution compliance: add configurable safe retries

Problem

The constitution requires configurable retries with safe defaults: idempotent operations may retry by default, while non-idempotent operations require explicit opt-in. The SDK currently delegates to vendor defaults without a portable or provider-specific configuration contract.

Scope

• Define client-level retry configuration and operation idempotency classification.
• Map supported settings to Azure Cosmos, AWS DynamoDB, and Google Spanner retry policies while preserving vendor behavior that cannot be normalized.
• Define attempt/backoff/throttling diagnostics and validation.
• Keep provider-specific controls explicit and portable defaults safe.

Acceptance criteria

• Retry limits/backoff are configurable for every provider.
• Non-idempotent operations are not retried unless explicitly enabled.
• Defaults and unsupported provider-specific settings are documented and tested.
• Diagnostics expose provider-supplied throttling/backoff and retry attempts where available.

Begin with the client-level configuration and provider integration entry points for Azure Cosmos, AWS DynamoDB, and Google Spanner; no files or tests are named in the issue. Compare current vendor retry behavior and define the portable settings, idempotency rules, diagnostics, validation, and documentation needed to satisfy the acceptance criteria.