fix(sse): replace synthesized kiro "Continue" turn with neutral filler (#5231)

[diegosouzapw]

Closes #5231

Problem

open-sse/translator/request/openai-to-kiro.ts (convertMessages) requires currentMessage to be a user turn. When an OpenAI→Kiro request ends on an assistant/tool turn (no trailing user turn to promote), the translator synthesizes one with literal content "Continue". Kiro/CodeWhisperer can read the word "Continue" as a real user instruction and take unintended agent action.

Fix

Replace the synthesized filler "Continue" → "..." (neutral). Single-string behavioral change at the synthesized trailing-turn site (openai-to-kiro.ts:415).

Scope (intentionally narrow)

• A trailing tool-result turn is still promoted as-is — it already collapses to a real user turn (carrying its toolResults), so it never hits the synthesized-filler branch. Only the assistant-text-ending case changes.
• No other behavior touched.

TDD / Validation (Hard Rule #18)

• Updated the existing assertion in tests/unit/translator-openai-to-kiro.test.ts (Continue → neutral filler) — failed on the unfixed code, passes after the fix.
• New dedicated guards tests/unit/kiro-continue-filler-5231.test.ts:
  • assistant-text-ending request never leaks the literal "Continue" filler;
  • a trailing tool-result turn is promoted as-is (carries its tool results), proving the change is surgically scoped.
• node --test (kiro suites): 31/31 pass.
• typecheck:core clean, eslint 0 errors, check:file-size clean (source kept net-neutral under the frozen cap; new tests in a fresh file), check:test-discovery OK.

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