refactor: extract shared streaming utilities module

Agent: carmack

Create crates/g3-providers/src/streaming.rs with shared helpers:
- decode_utf8_streaming(): Handle incomplete UTF-8 sequences in SSE streams
- is_incomplete_json_error(): Detect incomplete vs malformed JSON
- make_final_chunk(): Create finished completion chunks
- make_text_chunk(): Create text content chunks
- make_tool_chunk(): Create tool call chunks

Refactor anthropic.rs:
- Use shared decode_utf8_streaming (removes 15 lines of inline UTF-8 handling)
- Use make_final_chunk, make_text_chunk, make_tool_chunk helpers
- Reduces verbose CompletionChunk constructions throughout

Refactor databricks.rs:
- Remove local copies of streaming helpers (now uses shared module)
- Reduces duplication between providers

Net reduction: 118 lines removed, 16 lines added (including new module)
All tests pass. Behavior unchanged.

crates/g3-providers/src/lib.rs

@@ -1,3 +1,6 @@
+mod streaming;
+pub use streaming::{decode_utf8_streaming, is_incomplete_json_error, make_final_chunk, make_text_chunk, make_tool_chunk};
+
 use anyhow::Result;
 use rand::Rng;
 use serde::{Deserialize, Serialize};