Implement updated standard retry behavior

[Alan4506]

Description of changes:

Updates the standard retry behavior:

• Throttling backoff: throttling errors use a larger base backoff (1000ms) than other errors (50ms), based on error.is_throttling_error.
• Retry quota: RETRY_COST is now 14, throttling retries cost 5, and the timeout-specific cost is removed.
• DynamoDB defaults: DynamoDB / DynamoDB Streams clients default to 4 max attempts and a 25ms base backoff, applied per-value (a value configured by the customer always wins). The generic runtime stays AWS-agnostic; the DynamoDB knowledge lives in an AWS codegen integration.
• Long-polling: long-polling operations back off before giving up even when the quota is exhausted. Detection checks the smithy.api#longPoll trait at runtime through APIOperation.long_polling.
• x-amz-retry-after: honored when returned by the service, capped at 5 seconds above the normal backoff; invalid/missing values fall back to normal backoff and the standard HTTP Retry-After is ignored.

Note on long-polling detection:

Some service models don't apply the smithy.api#longPoll trait yet. We deliberately do not hard-code those operations. Doing this would require codegen interface changes for temporary behavior. The trait lookup is the long-term design regardless. Until those service models apply the trait, those operations won't get long-polling backoff.

Testing:

Added unit tests for each change (throttling vs non-throttling backoff, quota costs, the quota-exhaustion backoff contract, and x-amz-retry-after parsing/clamping/wiring). Also updated affected functional tests. All Python checks pass (make check-py, make test-py) and the Java codegen builds successfully.

Follow-up:

The awsJson protocol's _create_error is not on develop yet (it lives on the json-rpc branch). Once that merges, it needs the same parse_retry_after(response) wiring that awsQuery already has, so x-amz-retry-after is honored for awsJson services (e.g. DynamoDB) too.

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[arandito]

Thanks @Alan4506! I left a couple comments. My main concerns related to how we are detecting long polling operations and simplifying the StandardRetryStrategy constructor. Let me know if you have any questions!

[arandito]

LGTM

Another reviewer has reviewed the changes.