perf(misk-aws2-dynamodb): truncate tables on external fixture reset instead of drop/recreate

[rnc505]

Background

• ExternalTestDynamoDbClientModule currently drops and recreates every configured table before each test.
• Against a shared DynamoDB Local server, repeated table DDL can add substantial fixed latency to every test.
• In early testing against a service suite with thousands of tests, truncating rows instead reduced the test phase from approximately 17 minutes to approximately 7 minutes.

Overview of Changes

• Recreate configured tables once per injector to remove state left by earlier processes.
• Truncate rows on later resets using projected primary keys and batched deletes.
• Handle composite and reserved key names, scan pagination, and bounded retries for unprocessed writes.
• Preserve the module's existing public API and binding layout.
• Add ResetStrategy.DROP_RECREATE as a one-line opt-out for consumers using DynamoDB Streams or mid-test table DDL, whose state cannot be isolated by truncating rows.
• Add coverage for initial state, more than 25 rows, unchanged table identity, GSI cleanup, and drop/recreate resets.

Testing Performed

• bin/gradle :misk-aws2-dynamodb:test --warn
• bin/gradle :misk-aws2-dynamodb:check --warn