Part of #1592

Scope

Add SSE streaming support for Gemini's streamGenerateContent endpoint.

Files to Modify

• crates/zeph-llm/src/gemini.rs -- implement chat_stream(), update supports_streaming()
• crates/zeph-llm/src/sse.rs -- add gemini_sse_to_stream() parser

Key Implementation Details

• Streaming endpoint: POST /v1beta/models/{model}:streamGenerateContent?key={API_KEY}&alt=sse
• Each SSE data: event contains a partial JSON response with full candidate structure
• Text is delivered incrementally in candidates[0].content.parts[0].text
• Gemini 2.5 models emit thinking content as parts with thought: true flag -- map to StreamChunk::Thinking
• Need to handle both text-only and thinking+text streaming

Acceptance Criteria

• supports_streaming() returns true
• chat_stream() produces correct StreamChunk::Content items
• Thinking content emitted as StreamChunk::Thinking for 2.5 models
• SSE connection errors handled gracefully with retry
• status_tx events emitted on retry
• Unit tests with wiremock SSE mock responses