Fix Anthropic streaming crash on non-hash SSE payloads

[crmne]

What this does

Prevents streaming parsers from passing non-Hash SSE payloads into provider build_chunk methods.

handle_data can parse valid JSON scalars (like true) from data: frames. Those values are truthy and previously passed if data, which crashes provider chunk builders that assume a Hash and call dig.

This change narrows the guard in handle_stream to data.is_a?(Hash).

Why this is minimal

• One-line runtime behavior change in shared streaming handler
• No provider-specific branching
• Keeps existing behavior for all normal Hash event payloads

Tests

• Added spec/ruby_llm/streaming_spec.rb
• Verifies non-Hash SSE payloads are ignored without raising
• Verifies Hash SSE payloads still flow to build_chunk

Fixes #656

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.38%. Comparing base (afe7d04) to head (c98a4a7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #657   +/-   ##
=======================================
  Coverage   81.38%   81.38%           
=======================================
  Files         116      116           
  Lines        5430     5430           
  Branches     1428     1428           
=======================================
  Hits         4419     4419           
  Misses       1011     1011           

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