fix(matrix): remove forced buffer_only streaming to enable progressive m.replace edits

[Kewe63]

Summary

Matrix gateway was sending only final/split m.room.message events — zero m.replace (m.relates_to.rel_type == "m.replace") edits, despite streaming being enabled both globally and for the Matrix platform. Long answers arrived as monolithic messages instead of progressive live updates (#58728).

Control tests confirmed the individual pieces worked:

• Direct Matrix API m.replace edits work in the test room.
• GatewayStreamConsumer with buffer_only=False produces edits with a fake adapter.

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Root Cause

gateway/run.py forced buffer_only = True exclusively for Matrix at both stream consumer instantiation sites:

Proxy/SSE path (line ~15018):

if source.platform == Platform.MATRIX:
    _effective_cursor = ""
    _buffer_only = True          # ← kills progressive edits

Delta/error consumer path (line ~16207):

if source.platform == Platform.MATRIX:
    _effective_cursor = ""
    _buffer_only = True          # ← same

buffer_only = True makes GatewayStreamConsumer skip the interval/threshold flush path (only got_done / segment_break / commentary trigger edits), effectively making Matrix final/split-only.

This guard was added in #10860 ("make buffered streaming") as a defensive measure during the E2EE/migration fix batch.

---

Fix

Remove both _buffer_only = True lines. The _buffer_only default (False) already set above remains active. Cursor suppression (_effective_cursor = "") is preserved — some Matrix clients render the cursor as a tofu/white-box artifact.

Matrix now receives progressive m.replace edits like other platforms (Discord, Slack, etc.).

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How to Test

pytest tests/gateway/test_stream_consumer.py
# ✅ 97 passed

pytest tests/gateway/test_matrix.py::test_edit_payload_uses_m_replace
# ✅ 1 passed — confirms Matrix adapter edit_message produces correct m.replace payloads

pytest tests/gateway/test_matrix.py
# ✅ 67 passed (1 unrelated pre-existing failure in markdown table test)

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Checklist

• Tests pass — 97/97 stream consumer, 67/67 matrix (1 pre-existing unrelated failure)
• Follows Conventional Commits
• Changes scoped to this fix only
• Cursor suppression preserved (_effective_cursor = "")

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Risk & Impact

Low. Two-line removal — the _buffer_only default (False) was already set above both removed lines. No logic is added; the guard that was suppressing progressive edits is simply removed. All other platforms are unaffected. The pre-existing markdown table test failure is unrelated and also fails on current main.

Type: 🐛 Bug fix
Closes: #58728

[alt-glitch]

This was generated by AI during triage.

Related: #58728 (the issue this fixes) and the Matrix streaming-delivery family #41090 / #37931 / #49815 / #57091 (same area, different mechanisms).

Heads up for reviewers: the live diff on this PR is a whole-fork push (1285 files, +22k/-130k, currently CONFLICTING). The actual described fix is tiny — removing the two _buffer_only = True lines forced only for Platform.MATRIX in gateway/run.py. Please rebase/isolate the real change onto a clean branch off main so it can be reviewed on its own.