fix(threads): stop persisting legacy fallback threads when AI failed

[m-szymanska]

Problem

When the AI runtime is unavailable (dead/missing API key), the legacy assistive fallback persisted every failed attempt as a conversation thread with an "AI Failed" assistant message. After the thread-heal fix (#49) these junk threads became visible, cluttering the history rail — including repeated recordings of the same utterance producing 3-4 identical junk threads.

Fix

• Extracted legacy_fallback_assistant_text(status, text) -> Option<String>: Failed/Skipped → None (no thread persisted, warn! logged), Applied/AiNoop → Some(text) (persisted as before — partial success unaffected).
• No retry-loop dedup needed: verified there is no per-attempt retry in code; the observed duplicates were separate recordings on a dead key, all extinguished at the source by this change.

Tests

• New unit test legacy_fallback_skips_persist_on_failed_status covering all four status arms.
• Gates: cargo fmt --check, cargo clippy --all-targets -- -D warnings, cargo test -p codescribe (239 passed), cargo test -p codescribe-core (645 passed) — all green.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request introduces a helper function legacy_fallback_assistant_text to prevent persisting failed or skipped legacy formatter attempts as threads, addressing an issue where failed attempts resulted in duplicate garbage threads. It also adds corresponding unit tests. The review feedback suggests simplifying a redundant condition in run_legacy_send_path where assistant_text.is_none() is checked alongside the Failed status.

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[gemini-code-assist]

The check assistant_text.is_none() is redundant. Since legacy_fallback_assistant_text is guaranteed to return None when the status is AiFormatStatus::Failed, checking status == crate::ai_formatting::AiFormatStatus::Failed is sufficient. Simplifying this condition improves code readability.

Suggested change

	if assistant_text.is_none() && status == crate::ai_formatting::AiFormatStatus::Failed {
	warn!(
	"Legacy formatter failed; skipping thread persist for this attempt (a failed attempt is not a conversation)"
	);
	}
	if status == crate::ai_formatting::AiFormatStatus::Failed {
	warn!(
	"Legacy formatter failed; skipping thread persist for this attempt (a failed attempt is not a conversation)"
	);
	}