fix(alerts): Stronger typing around human_desc

[kcons]

There were previously exceptions due to type issues that should've been easy to catch statically here, so it's worth being explicit with the typing and simplifying where we can to avoid such issues recurring.

I aimed to minimize behavior changes, but found 4 pre-existing bugs in the process that addition type refinements and simplifications made visible to tools, so fixed those too.

[saponifi3d]

typppeeesss! (thanks for keeping this a smaller focused pr!) lgtm.

[sentry]

Bug: The assert in WorkflowEngineActionSerializer will cause a crash when serializing workflow actions with types like GITHUB or JIRA, as they are not supported by ActionService.
_{Severity: HIGH}

Suggested Fix

Instead of asserting, the serializer should gracefully handle unsupported action types. This could involve filtering them out from the serialized output or logging a warning and skipping the problematic action, which would prevent the API endpoint from crashing.

Prompt for AI Agent

Review the code at the location below. A potential bug has been identified by an AI
agent. Verify if this is a real issue. If it is, propose a fix; if not, explain why it's
not valid.

Location: src/sentry/incidents/endpoints/serializers/workflow_engine_action.py#L58

Potential issue: The `WorkflowEngineActionSerializer` uses an `assert` to validate that
the `obj.type` of a `workflow_engine.Action` is a valid `ActionService`. However, the
`Action.Type` enum allows for types like `GITHUB`, `JIRA`, `WEBHOOK`, and `PLUGIN`,
which are not defined in `ActionService`. When `ActionService.get_value()` is called
with one of these unsupported types, it returns `None`. This triggers the `assert
type_value is not None` statement, raising an `AssertionError` and crashing the API
endpoint responsible for serializing metric alert rules. This will prevent users with
alert rules configured with these action types from viewing or managing them.

_{Did we get this right? 👍 / 👎 to inform future reviews.}

[kcons]

ok, valid point.