fix: add helpful error message for malformed tool JSON in non-beta streaming path

[Krishnachaitanyakc]

Summary

• Wraps the bare from_json() call in the non-beta streaming accumulator (_messages.py) with a try-except ValueError that produces a clear, actionable error message — matching the existing behavior in the beta path (_beta_messages.py)

Problem

When using client.messages.stream() with tools, if the model emits malformed JSON during an input_json_delta event, the user gets a raw internal parser error with no context:

ValueError: expected ident at line 1 column 11

The beta path (_beta_messages.py) already wraps the same call and produces a helpful message:

ValueError: Unable to parse tool parameter JSON from model. Please retry your request or adjust your prompt. Error: expected ident at line 1 column 11. JSON: {"city": INVALID_VALUE}

The non-beta path was simply missing the same try-except.

Changes

• src/anthropic/lib/streaming/_messages.py: Added try-except ValueError around the from_json(json_buf, partial_mode=True) call in accumulate_event(), re-raising with a descriptive message that includes the original error and the raw JSON that failed to parse

Test plan

• All 21 existing streaming tests pass (tests/test_streaming.py)
• All 25 streaming lib tests pass (tests/lib/streaming/)
• Zero behavior change for valid JSON — the try-except only activates on malformed input
• Verified the error message format matches the beta path exactly

Fixes #1265