#6502: fix(tui): skip empty text for tool-only assistant turns

Tool-only turns showed "(no output)" in the TUI, confusing users during multi-step operations. Return empty string from resolveFinalAssistantText and guard finalizeAssistant with if(finalText), matching the pattern in tui-session-actions.ts. Fixes #3361 🤖 AI-assisted: Built with Claude Code (claude-opus-4-5) Testing: Fully tested - lint, build, and all tests pass The contributor understands and has reviewed all code changes. <!-- greptile_comment --> <h2>Greptile Overview</h2> <h3>Greptile Summary</h3> This PR adjusts TUI rendering for assistant turns that contain no user-visible text (e.g., tool-only runs). `resolveFinalAssistantText` now returns an empty string instead of the literal "(no output)", and the chat event handler skips calling `chatLog.finalizeAssistant` when the finalized text is empty, preventing confusing placeholder output during multi-step operations. These changes fit into the existing TUI streaming/finalization flow (`TuiStreamAssembler` → `ChatLog` updates) by treating “no content” as “render nothing”, consistent with other guards in the TUI that already skip empty delta updates. <h3>Confidence Score: 5/5</h3> - This PR is safe to merge with minimal risk. - The change is small, localized to TUI formatting/finalization, and aligns with existing patterns (skipping empty delta updates). It should only suppress a placeholder string and avoid finalizing empty assistant messages; no behavioral changes to tool streaming or error handling were introduced. - No files require special attention <!-- greptile_other_comments_section --> **Context used:** - Context from `dashboard` - CLAUDE.md ([source](https://app.greptile.com/review/custom-context?memory=fd949e91-5c3a-4ab5-90a1-cbe184fd6ce8)) - Context from `dashboard` - AGENTS.md ([source](https://app.greptile.com/review/custom-context?memory=0d0c8278-ef8e-4d6c-ab21-f5527e322f13)) <!-- /greptile_comment -->