Fix ideation backend reliability regressions

[adriandemian]

Context
DB fact-checking the failed ideation sessions showed four backend/runtime regressions: parent ideation calls were guided to omit session_id for get_plan_verification, stale registered Git worktrees were not pruned/retried, useful review transcripts could be polluted by a later provider stderr note, and split/timeline finalization could leave blank legacy chat message rows.

Impact
This unblocks the ClickUp-style parent ideation verification path, makes retry worktrees recover from Git's missing-but-registered state, avoids misleading post-success review errors, and reduces empty comment rows in tool-heavy chat/session timelines.

Decisions

• Keep verifier-child behavior intact while allowing parent ideation callers to pass session_id.
• Handle the exact missing-registered worktree fatal with prune + one retry in checkout_existing_branch_worktree.
• Suppress generic review error notes only after the task has already left Reviewing and useful content exists.
• Backfill no-output placeholders during split/timeline finalization instead of leaving blank legacy content.

Risks
The no-output placeholder backfill is intentionally conservative, but split-message rendering still deserves a broader integration test pass.

Validation

• npm test -- --run src/tests/tools.test.ts
• cargo test -p ralphx --lib test_checkout_existing_branch_worktree_retries_after_missing_registered_entry
• cargo test -p ralphx --lib test_post_success_review_error_note_suppression_only_after_reviewing

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[codecov]

Codecov Report

❌ Patch coverage is 76.59574% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
.../application/chat_service/chat_service_handlers.rs	72.72%	6 Missing ⚠️
src-tauri/src/application/git_service/worktree.rs	66.66%	5 Missing ⚠️

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[adriandemian]

Superseded by combined frontend/backend fix in #415.