fix(gateway): inject newline separator at stream iteration boundaries

[sprmn24]

What does this PR do?

Fixes a bug where resumed streamed text after tool calls was concatenated directly onto prior content with no separator, making multi-iteration responses unreadable.

Related Issue

Closes #2177

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✅ Tests (adding or improving test coverage)

Changes Made

• gateway/stream_consumer.py: Updated on_delta() to treat None as an iteration boundary signal. When None is received and the buffer does not already end with a newline, a separator is injected into the queue.
• tests/gateway/test_stream_consumer.py: New test file — 8 tests covering the boundary injection logic, including the exact regression case from this issue.

No changes to run_agent.py — it already emits stream_delta_callback(None) at the right point; the consumer just wasn't handling it.

How to Test

1. Enable gateway streaming (streaming.enabled: true in config.yaml)
2. Send a message that triggers a tool call mid-response (e.g. "search for X and summarize")
3. Observe that resumed text after the tool call starts on a new line
4. Run unit tests: python3 -m pytest tests/gateway/test_stream_consumer.py -v

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes
• I've tested on my platform: Ubuntu 24.04

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Screenshots / Logs

8/8 tests passing: python3 -m pytest tests/gateway/test_stream_consumer.py -v

[sprmn24]

I created a new PR without realizing that there were already PRs created previously.
I noticed there are similar PRs (#2186, #2187) addressing this issue.
My approach handles the boundary signal in on_delta() with 8 unit tests.
Happy to collaborate or close this if another PR is preferred.

[teknium1]

Closing — the streaming linebreak issue was fixed via PR #2473 (deferred flag approach from #2433). Thanks!