Observe subscription pump task failures

[beiwei30]

What changes

Observe connection-owned subscription event pump tasks through completion so panics and unexpected cancellations cannot be silently discarded. Expected owner teardown remains quiet, while abnormal exits emit one fixed diagnostic through the existing terminal stream error path.

The change centralizes pump registration and ownership for turn, background, and goal-continuation subscriptions, and adds lifecycle coverage for normal EOF, panic, unexpected cancellation, and processor drop.

Verification

• cargo fmt --all --check — passed
• cargo clippy --workspace --all-targets -- -D warnings — passed
• cargo check --workspace — passed
• cargo test -p orbcode-app-server message_processor — 40 passed
• cargo test -p orbcode-app-server processor_drop_aborts_active_subscriptions — passed
• cargo test -p orbcode-app-server — 234 unit tests plus all integration and doc tests passed
• RUST_TEST_THREADS=1 cargo test --workspace — all workspace and doc tests passed
• scripts/check-docs.sh — passed
• scripts/audit-public-surface.sh — passed
• scripts/audit-brand.sh — passed
• git diff --check — passed

Parallel workspace runs exposed existing load-sensitive startup/timeout flakes in unrelated tests. Each affected exact test passed three consecutive reruns; the full serialized workspace run passed.

Compatibility

• No golden fixture under compat-fixtures/fixtures/ or tui/testdata/ changed.
• No TypeScript-CLI compatibility name was renamed; scripts/audit-brand.sh passes.
• No public API changed; scripts/audit-public-surface.sh passes.
• Behavioural change is covered by tests in orbcode-app-server.

Notes for the reviewer

Scope follows the checked-in P3-01 inventory: only connection-owned active subscription pumps in app-server/src/message_processor.rs are changed. Pending request waiters, client transports, core turns, and TUI task families remain deferred and untouched.

[Copilot]

Copilot was unable to review this pull request because the user who requested the review has reached their quota limit.