fix(accumulator): sanitize invalid JSON in Input before marshaling

[jkakar]

Problem

Message.Accumulate() fails with JSON marshaling errors when processing streaming responses where InputJSONDelta events produce truncated or malformed JSON. This happens because:

1. InputJSONDelta events accumulate partial JSON strings into cb.Input (a json.RawMessage) by byte concatenation
2. At ContentBlockStopEvent and MessageStopEvent, json.Marshal is called to populate JSON.raw for AsAny()
3. Go's json.Marshal validates json.RawMessage contents via json.Compact, so if the accumulated bytes form invalid JSON, the marshal fails

This produces errors like:

• json: error calling MarshalJSON for type json.RawMessage: unexpected end of JSON input
• json: error calling MarshalJSON for type json.RawMessage: invalid character 's' after object key

The error is intermittent and depends on specific API response patterns where partial JSON chunks don't assemble into valid JSON (stream interruptions, network issues, etc.).

Solution

Add a sanitizeInputJSON helper that checks json.Valid() on the accumulated Input before marshaling. If the JSON is invalid, it replaces the bytes with null. This allows Accumulate() to succeed gracefully — callers can detect the issue by checking for a null Input on tool_use blocks.

The fix is applied at both ContentBlockStopEvent (per-block) and MessageStopEvent (all blocks) in both messageutil.go and betamessageutil.go.

Test plan

• Added test case: tool use block with truncated input JSON ({"argument": without closing)
• Added test case: tool use block with malformed input JSON ({"key s)
• Both cases verify that Accumulate() succeeds and the resulting Input is null
• All 12 existing TestAccumulate cases continue to pass

Fixes #255

[jkakar]

I'm not convinced this change is actually useful, because it doesn't address the core problem of why JSON is malformed, but I run into this periodically and have had to introduce defensive code in my application to deal with this problem so I decided to push this fix. This code was generated by Claude, and I've read and reviewed it but my overall understanding of the SDK is somewhat shallow so please apply skepticism as you read and contemplate whether incorporating this is useful or not.