fix(channels): ensure newline between narration and draft status lines

[theonlyhennygod]

Summary

• Base branch target: master
• Problem: In draft-capable channels (Telegram), native tool-call narration text that doesn't end with \n gets concatenated directly with the next status line, producing garbled output like Task started.⏳ count_to
• Why it matters: User-facing draft messages become unreadable when narration and status lines run together
• What changed: The narration delta sent via on_delta now always ends with \n before being dispatched to the draft updater
• What did not change: Non-draft channels, progress line format, final-answer streaming, console output

Label Snapshot (required)

• Risk label: risk: low
• Size label: size: XS
• Scope labels: agent
• Module labels: agent: loop
• Contributor tier label: N/A
• If any auto-label is incorrect, note requested correction: N/A

Change Metadata

• Change type: bug
• Primary scope: agent

Linked Issue

• Closes [Bug]: Telegram draft updates concatenate native tool narration with status lines #4348

Validation Evidence (required)

cargo fmt --all -- --check   # pass
cargo check --lib            # pass
cargo test run_tool_call_loop_relays_native_tool_call_text_via_on_delta   # pass

• Existing test updated to match new trailing-newline behavior.

Security Impact (required)

• New permissions/capabilities? No
• New external network calls? No
• Secrets/tokens handling changed? No
• File system access scope changed? No

Privacy and Data Hygiene (required)

• Data-hygiene status: pass
• Redaction/anonymization notes: N/A
• Neutral wording confirmation: Yes

Compatibility / Migration

• Backward compatible? Yes
• Config/env changes? No
• Migration needed? No

Human Verification (required)

• Verified scenarios: Reviewed the delta accumulation path; narration text without trailing newline now gets one appended before dispatch
• Edge cases checked: Narration text that already ends with \n (no double newline — guarded by ends_with check); empty display_text (guarded by outer !display_text.is_empty())
• What was not verified: Live Telegram draft rendering

Side Effects / Blast Radius (required)

• Affected subsystems/workflows: Draft message rendering in channels that support supports_draft_updates()
• Potential unintended effects: An extra blank line could appear if the model's narration already ends with \n — mitigated by the ends_with guard
• Guardrails/monitoring for early detection: Existing test updated to assert trailing newline

Rollback Plan (required)

• Fast rollback command/path: Revert this single commit
• Feature flags or config toggles: None
• Observable failure symptoms: Narration and status lines concatenated without separator in Telegram drafts

Risks and Mitigations

• Risk: None — the change is a one-line guard adding a newline character.