docs(sdk): clarify Responses streaming and tokenizer boundaries

[vedjoshi1]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

This accompanies SDK issue #4976
and the SDK implementation PR.

• Document Responses API streaming through responses() and aresponses(),
  callback behavior, and final completion selection.
• Describe the private provider boundaries and optional chat-template tokenizer
  fallback in the SDK architecture page.
• Remove the outdated statement that only Chat Completions supports streaming.

Verification

uv run --with pytest --with requests --with pyyaml pytest -q tests/ produced
35 passed and 2 unrelated setup errors. Both errors are from the existing
pricing tests fetching OpenHands/OpenHands/main/openhands/utils/llm.py, which
currently returns HTTP 404. No pricing files or test fixtures changed.

The paired SDK branch passed 1,073 LLM/checker tests, 6,432 SDK tests, and six
local real HTTP/SSE cases. It also built and started the packaged Agent Server.