fix: add missing JSON error handling in non-beta streaming accumulator

[AbdoKnbGit]

Fixes #1265

The beta streaming accumulator (_beta_messages.py) wraps from_json() in a
try-except that gives users a clear, actionable error when the model emits
malformed tool input JSON. The non-beta accumulator (_messages.py) was missing
the same handler — causing users to see raw jiter errors with no context.

This PR adds the matching try-except to the non-beta path and a test mirroring
the existing beta test.

---

Changes

• src/anthropic/lib/streaming/_messages.py — wrap from_json() at L480
  in a try-except with a helpful error message, matching _beta_messages.py L500-510
• tests/lib/streaming/test_partial_json.py — add
  test_partial_json_with_invalid_json_non_beta, mirroring the existing beta test

---

Before / After

Before — non-beta path, malformed tool JSON:

ValueError: expected ident at line 1 column 11

No context. No mention of tool JSON. Looks like an SDK crash.

After — matches beta behavior:

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}

---

Test plan

• New test test_partial_json_with_invalid_json_non_beta passes
• All 26 streaming tests pass (25 existing + 1 new)
• Confirmed with real API call against claude-haiku-4-5
• Zero behavior change for valid JSON

[AbdoKnbGit]

@claude @karpetrosyan hi can you review this