fix(gateway): use buffer_only mode for Photon to prevent streaming cursor tofu

[liuhao1024]

What does this PR do?

Adds Photon (iMessage) to the buffer_only streaming mode check so the streaming cursor character is suppressed. Previously, the cursor rendered as a visible white-square/tofu artifact in iMessage bubbles.

Related Issue

Fixes #49793

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• gateway/run.py: Add Photon to the buffer_only=True condition in both streaming code paths (the _run_agent path and the inline-streaming path), alongside the existing Matrix check. Uses getattr(source.platform, "value", "") == "photon" since Photon is a plugin platform without an explicit enum member.
• tests/gateway/test_run_progress_topics.py: Add test_run_agent_photon_streaming_buffer_only regression test verifying no streaming cursor artifact and no intermediate edits for Photon sessions.

How to Test

1. Configure a Photon/iMessage gateway adapter
2. Send a message that triggers streaming response
3. Verify no white-square characters appear in the iMessage bubble
4. Run pytest tests/gateway/test_run_progress_topics.py::test_run_agent_photon_streaming_buffer_only -xvs
5. Run pytest tests/gateway/test_run_progress_topics.py::test_run_agent_matrix_streaming_omits_cursor -xvs (regression: Matrix still works)
6. Run pytest tests/gateway/test_stream_consumer.py::TestBufferOnlyMode -xvs (regression: buffer_only mode tests still pass)

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 (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS

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) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Code Intelligence

• Analyzed: gateway/run.py lines 13947-13957 and 15114-15124 (streaming buffer_only logic)
• Blast radius: LOW — additive condition to existing platform check, no control flow change
• Related patterns: Matrix buffer_only (same mechanism, line 13949/15115), StreamConsumerConfig buffer_only mode

[teknium1]

Thanks for the focused Photon streaming fix. Current main still defaults Photon to editable behavior at gateway/run.py:16751-16756 and gateway/run.py:18079-18089, so the two affected stream-consumer construction sites remain live.

Problems

• The added comments say buffer-only mode produces a final plain-text-only message. GatewayStreamConsumer still flushes on segment breaks and commentary (gateway/stream_consumer.py:625-630), while Photon preserves markdown by default (plugins/platforms/photon/adapter.py:1384-1390, 1481-1484).

Suggested changes

• Reword the two comments to describe cursor suppression and the absence of interval/threshold-driven updates, without promising forced plain text or universally final-only output.

The surrounding code has moved, but the two-hunk fix remains mechanically salvageable. Automated hermes-sweeper review.

[teknium1]

buffer_only suppresses interval/threshold flushes, but it still flushes on segment breaks and commentary; Photon also retains markdown by default. Please reword the surrounding comment to avoid promising a universally final-only plain-text message.

[teknium1]

Closing in favor of #62172 (same author) — the SUPPORTS_MESSAGE_EDITING capability flag is the convention-correct mechanism and landed via #73560. Thanks for both takes on this bug.