events: add UnmarshalStreamImage and ToDynamoDBJSON helpers (#58)

[MukundaKatta]

Summary

Refs #58. Adds backwards-compatible converter helpers from events.DynamoDBStreamRecord to the AWS SDK's AttributeValue shape, without changing any existing public API.

Approach: backwards-compatible

Did not change DynamoDBStreamRecord field types. The recurring concern in the issue thread was the SDK v1 vs v2 split and the binary-size cost of pulling in the SDK. This PR introduces no new module deps — helpers convert via canonical DynamoDB JSON.

What's added

• events/dynamodb_helpers.go:
  • UnmarshalStreamImage(image map[string]DynamoDBAttributeValue, out interface{}) error — direct unmarshal-into-struct that mirrors the StackOverflow workaround referenced in the issue.
  • (DynamoDBAttributeValue).ToDynamoDBJSON() ([]byte, error) — render to canonical DynamoDB JSON.
  • ToDynamoDBJSONMap(map[string]DynamoDBAttributeValue) ([]byte, error).
  • (DynamoDBStreamRecord).ToDynamoDBJSON() ([]byte, error) — full envelope.
• events/dynamodb_helpers_test.go — 8 tests covering scalars, nested map/list, null, set types, testdata round-trip, and envelope omitempty behavior.

Test plan

• go build ./... clean.
• go vet ./events/... clean.
• go test ./events/... passes (existing + 8 new).
• No existing public API touched.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 73.91304% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.06%. Comparing base (815d21f) to head (40688d8).

Files with missing lines	Patch %	Lines
events/dynamodb_helpers.go	73.91%	13 Missing and 5 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #624      +/-   ##
==========================================
- Coverage   75.12%   75.06%   -0.06%     
==========================================
  Files          36       37       +1     
  Lines        1419     1488      +69     
==========================================
+ Hits         1066     1117      +51     
- Misses        274      287      +13     
- Partials       79       84       +5     

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[MukundaKatta]

The TestEnableSigterm failure on 1.15 / 1.21 looks unrelated. It's in the lambda/ package, this PR only adds new files in events/. The failure is the Docker-backed Lambda runtime timing out on init (Reset initiated: Timeout) which I've seen as flaky on the runners. Happy to rebase if a maintainer wants a re-run.