fix(reasoning): suppress partial tag tokens during autoparser warm-up

The C++ PEG parser needs a few tokens to identify the reasoning format
(e.g. "<|channel>thought\n" for Gemma 4). During this warm-up, the gRPC
layer was sending raw partial tag tokens to Go, which leaked into the
reasoning field.

- Clear reply.message in gRPC when autoparser is active but has no diffs
  yet, matching llama.cpp server behavior of only emitting classified output
- Prefer C++ autoparser chat deltas for reasoning/content in all streaming
  paths, falling back to Go-side extraction for backends without autoparser
  (e.g. vLLM)
- Override non-streaming no-tools result with chat delta content when available
- Guard PrependThinkingTokenIfNeeded against partial tag prefixes during
  streaming accumulation
- Reorder default thinking tokens so <|channel>thought is checked before
  <|think|> (Gemma 4 templates contain both)