fix(ollama): preserve multi-byte UTF-8 content split across stream chunks

[pacocartones]

Summary

• The Ollama-compatible streaming transform (transformToOllama in open-sse/utils/ollamaTransform.ts) decoded every upstream network chunk with a fresh new TextDecoder().decode(chunk) and no { stream: true }. A multi-byte UTF-8 character (CJK, emoji) split across a chunk boundary was decoded as two incomplete halves and turned into U+FFFD replacement characters in the NDJSON delivered to Ollama-format clients. The line-level buffer reassembles split SSE lines, but the corruption happens one layer below at byte decode, so it cannot recover it.
• Fixed by hoisting a single persistent TextDecoder and decoding with { stream: true }, so pending bytes carry across chunks. This matches the pattern already used by every other streaming transform/executor in open-sse/ — e.g. responsesTransformer.ts (which comments "persistent decoder with { stream: true } carries pending bytes between chunks"), stream.ts, and the *-web executors. ollamaTransform.ts was the only streaming transform not doing this.

Related Issues

• No linked issue — unreported bug found while reviewing the streaming transforms.

Validation

• Change type: routing (SSE stream transform)
• Focused test: tests/unit/ollama-transform.test.ts — the new case fails on the pre-fix code ('���好世界🌍', AssertionError) and passes after; full file 10/10 with the fix.
• npm run lint / full unit suite / coverage / build — not run locally (dependencies not installed); CI (docs: official 'golden path' contribution guide by change type #8329) runs them on this PR. The change is a 3-line decoder hoist with no new imports.
• Reconciled with the current active release base (branched from main tip d5dfcff); focused test rerun afterward.
• Production-code change includes a new automated test in this PR.

Tests Added Or Updated

• tests/unit/ollama-transform.test.ts — new test "transformToOllama preserves multi-byte UTF-8 content split across chunks": splits a CJK+emoji delta at a UTF-8 continuation byte (0x80–0xBF) across two enqueued chunks and asserts exact reassembly with no U+FFFD. Verified failing on the pre-fix code and passing after.

Coverage Notes

• Touches open-sse/utils/ollamaTransform.ts; the new test exercises the changed decode path (chunk-boundary reassembly) directly.

Reviewer Notes

• Behaviour change is limited to byte decoding; SSE line framing, tool-call aggregation, and error/non-SSE passthrough are unchanged.

[pacocartones]

CI note — none of the red checks are introduced by this change (an ollama stream-decoder fix touching open-sse/utils/ollamaTransform.ts + its test). Build was cancelled by the workflow's fail-fast, not a compile error (log: "The operation was canceled"). Vitest (MCP / autoCombo / UI components) fails on tests/unit/ui/use-provider-models-auto-fetch.test.tsx — a provider-models UI test this diff does not touch, and the job is green on main; it is flaky here (the run also logs ECONNREFUSED and a better-sqlite3 binding fallback). Protocol Clients E2E is advisory. Rebased onto current main (green) so the diff is minimal.