feat(lambda): declare the scope's available actions in its spec

[agustincelentano]

Declares the actions this scope offers inside its own spec, so the
scope_definition module can read them instead of carrying a hardcoded default.

Why

The list lived in the module, far from the scope that implements the actions, and
that distance let the two drift. The default listed kill-instances while the
containers scope ships kill-instance.json.tpl; the module built a URL from the
wrong name, got a 404, and the body 404: Not Found reached jq as if it were the
template:

jq: parse error: Expected string key before ':' at line 1

An error that says nothing about a missing file. With the list next to the
templates it names, adding or renaming an action is one change in one place.

What it adds

Fourteen actions, matching both the templates in lambda/specs/actions/ and the
specifications currently registered for this scope:

create-scope  update-scope  delete-scope
start-initial  start-blue-green  finalize-blue-green  switch-traffic
rollback-deployment  delete-deployment
adjust-provisioned-concurrency  adjust-reserved-concurrency  invoke
diagnose-deployment  diagnose-scope

Verified that the rendered spec parses as JSON and that every name has its
.json.tpl.

Merge order

Needs nullplatform/tofu-modules#458, which makes the module read this field.
Until that merges this changes nothing, and it is harmless on its own.

Worth noting the reverse is not true: that PR removes the module's default, so a
scope whose spec does not declare available_actions ends up with an empty list —
and an empty list in a for_each destroys every action specification. This scope
had fourteen at risk.

🤖 Generated with Claude Code

[sebastiancorrea81]

lgtm