feat: warn about ignored inputs and remove misleading defaults

[JamBalaya56562]

When deploying with package-type: Image, several Zip-only inputs
(runtime, handler, layers) are silently dropped: index.js gates
each of them with packageType === 'Zip' && ... before sending the
CreateFunction / UpdateFunctionConfiguration request. Today the action
neither warns about the dropped values nor stops echoing the action's
own defaults into the workflow log, so the user has no signal that
their runtime: nodejs20.x (etc.) has no effect on the deployed
container function.

This is the same shape of footgun the action already guards against
for s3-bucket / s3-key / source-kms-key-arn on the Image side
and for image-uri / code-artifacts-dir on the Zip side — those
inputs already produce explicit core.warning(...) lines. This commit
extends the same treatment to the remaining Zip-only / Image-only
inputs.

Changes:

• validations.js: in checkInputConflicts, emit a warning when
  handler, runtime, or layers is supplied with
  package-type: Image, and when image-config is supplied with
  package-type: Zip. Each warning explains why the input is being
  ignored.

• action.yml: remove default: 'index.handler' from handler and
  default: 'nodejs20.x' from runtime. These defaults are
  injected into every action invocation by GitHub Actions and end
  up echoed in the with: block of the run log, even when the
  user did not write them — which is the exact source of confusion
  reported by users seeing runtime: nodejs20.x in their Image
  deploy logs (where it has no effect). The Zip fallback behavior
  is preserved by validations.js, which already falls back to
  'index.handler' and 'nodejs20.x' when the input is empty —
  so existing Zip users that omit either input keep getting the
  same Lambda runtime/handler. The default values are now
  documented in the input descriptions instead.

Tests:

• Three new cases under tests/validations.test.js
  Input Conflict Warnings:
  • warns about handler/runtime/layers for Image
  • does not warn when those inputs are omitted on Image
  • warns about image-config for Zip

dist/index.js is rebuilt with npm run build (ncc 0.36.1) since
validations.js is bundled into it.