feat(streaming): generic streaming response helpers beyond SSE

[ZhuchkaTriplesix]

Context

• SSE helper exists (oxyroute/sse.py) using Granian response_stream.
• No generic streaming helpers for bytes/text/NDJSON beyond SSE.

Problem

Handlers that stream downloads, logs, AI tokens, or proxy responses must use low-level protocol APIs directly.

Proposed change

• Add oxyroute.streaming (or extend oxyroute/sse.py) with helpers such as:
  • stream_text(async_iter, ...)
  • stream_bytes(async_iter, ...)
  • optional stream_jsonl(async_iter, ...)
• Correct headers, error handling, and documentation for backpressure limits.

Files

• oxyroute/streaming.py (or sse.py), src/dispatch.rs if sentinel integration needed
• docs/, tests

Acceptance criteria

• Streaming text/bytes routes covered by tests.
• Existing SSE tests still pass.
• Docs describe error and backpressure behaviour.

Framework impact

Enables common streaming use cases without touching Granian protocol directly.