[vishwakt]

Summary

Changes

Base64Encoded started its chain with z.string() and called fromBase64 inside the transform. fromBase64 throws a TypeError on invalid base64, and Zod 4 does not catch exceptions thrown inside transforms, so safeParse threw a TypeError instead of returning { success: false }. This affected the helper directly, the built-in KinesisDataStreamSchema / KinesisEnvelope, parse(event, undefined, KinesisDataStreamSchema, true), and the decorator / Middy middleware in safeParse mode.

Through parse() it was worse: Zod's ~standard.validate catches the synchronous throw and falls back to safeParseAsync, returning a rejecting promise. parse() saw a Promise, threw the misleading "supports only synchronous validation" error, and never attached a handler to the rejected promise, so it surfaced as an unhandled rejection (Runtime.UnhandledPromiseRejection on the Lambda runtime).

• Guard Base64Encoded with z.base64() instead of z.string(), so invalid input becomes a normal Zod issue before the transform runs. This matches the existing CloudWatchLogsSchema and KinesisFirehoseRecordSchema, which already guard with z.base64() and were not affected.
• In parse(), attach a handler to the promise returned by an async validation before throwing, so a rejected promise from Zod's async fallback cannot leak. Dropped the /* v8 ignore */ on that branch since it is reachable, and covered it with a test.

The decorator and Middy safeParse paths route through the same schemas, so they are fixed at the source by the helper change.

Added tests:

• Base64Encoded(...).safeParse('not base64!!') returns a failed result instead of throwing.
• parse(event, undefined, KinesisDataStreamSchema, true) with a malformed kinesis.data returns { success: false } rather than throwing.
• A schema whose transform throws synchronously makes parse() throw the sync-only ParseError without leaking an unhandled rejection (the run fails if the handler is removed).

Verified each test fails against the pre-fix code, and the full parser unit suite (289 tests) passes with 100% coverage on parser.ts and helpers/index.ts.

Issue number: closes #5651

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