fix: use ephemeral mode in OrchestrationClient to prevent read-only container failures

[Br1an67]

Closes #19317

Background services (e.g. actions service) use OrchestrationClient, which calls create_app(). This triggers UI static directory creation via create_ui_static_subpath(), which fails with PermissionError in read-only containers (common in rootless/secure deployments). The exception is swallowed, causing automation actions to silently fail and messages to go to DLQ with no error logs.

Changes

1. Skip UI creation for OrchestrationClient (src/prefect/server/api/clients.py)

Pass ephemeral=True to create_app() in BaseClient.__init__. Background services use in-memory ASGI transport and don't need the UI — ephemeral=True skips UI directory creation, preventing PermissionError in read-only containers.

2. Add error logging for unexpected action failures (src/prefect/server/events/actions.py)

Catch and log unexpected exceptions in the action consumer before re-raising. This provides visibility when actions fail for reasons other than ActionFailed, instead of messages silently going to DLQ.

3. Regression test (tests/server/api/test_clients.py)

Verifies that OrchestrationClient creates the app in ephemeral mode.

Checklist

• This pull request references any related issue by including "closes <link to issue>"
• If this pull request adds new functionality, it includes unit tests that cover the changes
• If this pull request removes docs files, it includes redirect settings in mint.json.
• If this pull request adds functions or classes, it includes helpful docstrings.

[codspeed-hq]

Merging this PR will not alter performance

✅ 2 untouched benchmarks

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_{Comparing Br1an67:fix/19317-ha-readonly-automation (f5c8c29) with main (13e96b7)}