fix: prevent TransferEncodingError in Gemini/Claude SSE streams

[Stranmor]

Problem

HTTP clients using aiohttp/httpx (e.g. OpenWebUI) receive:

TransferEncodingError: 400, message='Not enough data to satisfy transfer length header.'

Root Cause

When SSE streams encounter upstream errors (connection drop, timeout), the Gemini handler and Claude SSE mapper use yield Err(...) inside async_stream::stream!. This Err propagates through Body::from_stream(), causing hyper to abort HTTP/1.1 chunked transfer encoding without sending the final 0\r\n\r\n terminator chunk.

HTTP clients that validate chunked encoding integrity then throw TransferEncodingError.

Fix

Convert stream errors from Err(...) to Ok(Bytes) containing SSE-formatted error content. This matches the pattern already used by the OpenAI streaming handler (line ~282 in streaming.rs), which correctly wraps errors as Ok(error_json_bytes).

The stream terminates cleanly with None, hyper sends the proper 0\r\n\r\n terminator, and the client receives a well-formed HTTP response with error details.

Changes

File	Before	After
handlers/gemini.rs	yield Err(format!("Stream error: {}", e))	yield Ok(Bytes::from(error_json_sse))
mappers/claude/mod.rs	yield Err(format!("Stream error: {}", e))	yield Ok(Bytes::from(error_event_sse))

Already correct: OpenAI handler wraps errors as Ok(Bytes) with JSON error payload + [DONE].

[Stranmor]

CI Failures — Pre-existing, Not Caused by This PR

Check Rust Code (fmt): The cargo fmt --check failures are trailing whitespace in files not modified by this PR (request.rs, claude.rs, thinking_utils.rs, protobuf.rs). These formatting issues exist on main and would fail on any PR.

Build Tauri App (macOS/Windows): failed to decode secret key: incorrect updater private key password — this is a CI secrets configuration issue (TAURI_SIGNING_PRIVATE_KEY), unrelated to code changes.

This PR only modifies 2 files (gemini.rs, claude/mod.rs) with a 6-line diff (2 deletions, 6 insertions).

[Stranmor]

Closing in favor of #2322 — it uses serde_json::json!() for proper JSON escaping instead of manual format!() + replace(), which is more robust.