test(batch): add Lambda Managed Instances isolation coverage

[karthiksenv]

Summary

Add real-platform Batch isolation coverage for Lambda Managed Instances. The suite exercises module-scoped standard SQS and asynchronous FIFO processors with overlapping invocations, checking that record handlers, partial failures, and failed FIFO groups remain scoped to their invocation.

Changes

• Add standard SQS and FIFO LMI suites with 30 concurrent invocations, a first-record peer barrier, one worker per execution environment, and concurrency set to 10.
• Check actual callback records and exact failure IDs across healthy, partially failed, and fully failed batches. Reuse FIFO group names across invocations to expose leaked failed-group state.
• Register Batch in the existing LMI workflow through test:e2e:lmi, keep ordinary E2E runs separate, and increase shared capacity to retain spare instance headroom for the expanded matrix. The two Batch variants deploy and tear down sequentially.
• Add a local test of the same handlers using the real InvokeStore library. Both variants pass with isolation and fail when isolation is temporarily disabled: standard SQS mixes failure IDs and FIFO skips healthy records.

Validation

• Workspace build, all workspace test type checks, and staged Biome checks pass.
• Full unit suite: 3,935 tests pass; statements, branches, functions, and lines all have 100% coverage. Run with npx vitest --run --maxWorkers=4 --testTimeout=30000 --exclude tests/unit/layer-publisher.test.ts --coverage --coverage.thresholds.100 tests/unit. The existing CDK stack-tagging test exceeds its default five-second timeout on this Windows machine; the longer timeout is a local command-line setting only.
• Both handlers bundle, and CDK synthesis passes with shared and ephemeral capacity providers; checked concurrency, worker count, scaling, and publication configuration.
• Real AWS execution is pending: this environment has no AWS credentials. A maintainer needs to run the existing E2E workflow on this branch to validate Node.js 22/24 and x86_64/arm64 on Lambda Managed Instances before merge.

Issue number: closes #5684

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[svozza]

Thanks for this! Can you reply to the issue this PR closes so I can assign it to you?

[karthiksenv]

Thanks! I've commented on #5684 so you can assign it to me.

[svozza]

Thank you for your work here, great to have LMI test coverage of this utility. Merged!

[powertools-for-aws-oss-automation]

Awesome work, congrats on your first merged pull request and thank you for helping improve everyone's experience!

[karthiksenv]

Thanks, @svozza! I appreciate the careful reviews and the help getting the tests into shape. Glad to have the Batch LMI coverage merged, and happy to make my first contribution here.