Show cloud startup failures before a conversation exists

Description

Show cloud startup errors immediately when no conversation exists, including private-image credential failures. Use the startup-model message only when no conversation-derived error is available.

Startup failures show “Cloud agent failed to start” and omit credits, even when a task ID exists or task metadata arrives later. Task status enrichment and continuation actions stay unchanged.

Linked Issue

Requested port of the startup-error UI changes from #16516. No separate issue; unrelated nextest and search-test changes are excluded.

• The linked issue is labeled ready-to-spec or ready-to-implement. This small follow-up is tracked by the request and source PR.
• Where appropriate, screenshots or a short video of the implementation are included below. No computer-use opt-in was given; verification uses regression tests and lint.

Testing

The new no-conversation regression failed before the fix (missing registry error). The tests cover startup failures with and without task IDs and preserve conversation-error precedence.

• cargo nextest run -p warp --no-fail-fast --test-threads 1 -E 'test(terminal::view::ambient_agent::model::tests) | test(terminal::view::shared_session) | test(queued_prompt) | test(cloud_mode_failed_keeps_queued_query)': 163 passed.
• cargo clippy -p warp --all-targets --tests -- -D warnings: passed.
• ./script/format and git diff --check: passed.
• I have manually tested my changes locally with ./script/run. Live-registry and manual UI verification were not performed.

Agent Mode

• Warp Agent Mode - This PR was created via Warp's AI Agent Mode

Changelog Entries for Stable

CHANGELOG-BUG-FIX: Fixed missing cloud agent startup error details, including private Docker image credential failures.