fix(dynamodb): propagate billing-mode changes through GSIs

[vieiralucas]

Summary

DynamoDB's UpdateTable zeroes every GSI's ProvisionedThroughput when a table flips to PAY_PER_REQUEST, and CreateTable returns {0, 0} for GSIs on PAY_PER_REQUEST tables regardless of whether the caller sent a ProvisionedThroughput block. fakecloud previously left stale capacity numbers on GSIs, defaulted missing throughput to {5, 5}, and dropped AttributeDefinitions sent alongside a GSI Create op. The Terraform provider's post-apply refresh plan then reported drift, which blocked four upstream TestAcc tests behind the "billing-mode transitions with GSI" gap.

Fix:

• parse_gsi / CreateTable: zero GSI ProvisionedThroughput when billing mode is PAY_PER_REQUEST; keep {5, 5} default only for PROVISIONED.
• UpdateTable: on PROVISIONED -> PAY_PER_REQUEST transition, zero the table and every GSI's throughput before applying index updates.
• UpdateTable GSI Create op: pick throughput based on the table's current billing mode, matching real AWS semantics.
• UpdateTable: merge new AttributeDefinitions into the table schema so a follow-up DescribeTable reflects GSI hash/range keys that were added alongside the Create op.

Removes these four entries from the DynamoDB tfacc deny list:

• TestAccDynamoDBTable_BillingMode_payPerRequestBasic
• TestAccDynamoDBTable_BillingModeGSI_payPerRequestToProvisioned
• TestAccDynamoDBTable_BillingModeGSI_provisionedToPayPerRequest
• TestAccDynamoDBTable_Disappears_payPerRequestWithGSI

Test plan

• Three new e2e regression tests in crates/fakecloud-e2e/tests/dynamodb.rs cover create + update paths (dynamodb_create_pay_per_request_with_gsi_zeroes_throughput, dynamodb_update_table_provisioned_to_pay_per_request_zeroes_gsi, dynamodb_update_table_gsi_create_on_pay_per_request_zeroes_throughput).
• Full cargo test -p fakecloud-e2e --test dynamodb passes (41/41).
• cargo test -p fakecloud-conformance --test dynamodb passes (30/30).
• All four previously-denied upstream TestAccDynamoDBTable_BillingMode* tests pass locally against fakecloud via go test ./internal/service/dynamodb/ -run ....
• cargo clippy --workspace --all-targets -- -D warnings clean.

---

Summary by cubic

Fixes DynamoDB billing-mode transitions so GSIs and table throughput follow AWS semantics, preventing Terraform drift. Unblocks four upstream TestAccDynamoDBTable_BillingMode* tests.

• Bug Fixes
  • CreateTable: zero GSI throughput on PAY_PER_REQUEST; keep {5,5} default only for PROVISIONED.
  • UpdateTable: on PROVISIONED → PAY_PER_REQUEST, zero table and all GSIs before index updates.
  • GSI Create (UpdateTable): set throughput based on current table billing mode; return {0,0} on PAY_PER_REQUEST.
  • Merge AttributeDefinitions provided with GSI Create into the table schema so DescribeTable shows new keys.
  • Added 3 e2e tests in fakecloud-e2e and removed 4 denylist entries in fakecloud-tfacc.

^{Written for commit 479ce56. Summary will update on new commits.}

[cubic-dev-ai]

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