fix: Prevent the DynamoDB store from failing when flags or segments exceed 1 MB

[jsonbailey]

Summary

The DynamoDB feature store raised NoMethodError: undefined method 'exclusive_start_key=' for an instance of Hash when a Query for one kind needed a second page. DynamoDB returns at most 1 MB per page, so this happened once all flags or all segments together exceeded about 1 MB. init failed on every stream connect, so the persistent store never updated. The bug has been present since the DynamoDB store was added in 5.5.0.

• Set the start key on the request Hash in both paging loops (get_all_internal and read_existing_keys).
• Fix the same mistake in the spec helper that clears the table.
• Add tests against DynamoDB Local. A client handler sets a small Query limit, so the real server pages a small data set. The tests check that more than one page is read in all and in init.

Verified against aws-sdk-dynamodb 1.174.0, and 1.57.0 (the gemspec floor) with aws-sdk-core 3.109.0 (its floor).

Closes #462

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Note

Overview
Fixes DynamoDB feature store pagination when a Query returns more than one page (~1 MB of flags/segments). Paging loops now set req[:exclusive_start_key] on the request Hash instead of calling exclusive_start_key=, which raised NoMethodError and broke all and init on the second page.

The spec table-clear scan loop uses the same Hash-key fix. New DynamoDB Local tests inject a small Query limit so multi-page reads are exercised for all and for init (including deletion of keys dropped from the new payload).

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