Allow ClientContext.Custom unmarshaling for non-string (JSON) values

[M-Elsaeed]

Issue #, if available: -

Description of changes:

When AWS services like Bedrock Agentcore Gateway send nested JSON objects as values in ClientContext.custom (e.g. propagated headers), the Go Lambda runtime fails with: cannot unmarshal object into Go struct field ClientContext.custom of type string. This blocks all Go Lambda functions used with Agentcore Gateway propagated headers.

Root cause
ClientContext.Custom is map[string]string, which rejects non-string JSON values during json.Unmarshal in parseClientContext() before the handler is ever invoked.

How this Fixes that
Add a custom UnmarshalJSON to ClientContext that parses Custom values via json.RawMessage. String values are stored directly (backward compatible) while Non-string values (objects, arrays) are serialized to their JSON string representation instead of failing.

Fully backward compatible - no struct signature changes. All existing tests pass.

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.99%. Comparing base (9c32960) to head (3de2206).

Files with missing lines	Patch %	Lines
lambdacontext/context.go	0.00%	18 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #620      +/-   ##
==========================================
- Coverage   74.94%   73.99%   -0.96%     
==========================================
  Files          36       36              
  Lines        1401     1419      +18     
==========================================
  Hits         1050     1050              
- Misses        273      291      +18     
  Partials       78       78              

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[maxday]

Maybe you can add a test when JSON parsing is failing? (I think a non-valid JSON will trigger that code path?)