SSM: resolve full parameter ARNs in get_parameters (#9396)

[balramthewarrior]

Partially addresses #9396.

Problem

get_parameter accepts a full parameter ARN — that was fixed in #7748. get_parameters was not updated to match, so reading parameters by ARN (as you do for a parameter shared from another account via RAM) silently returns nothing:

arn = f"arn:aws:ssm:{region}:{account}:parameter/Parameter1"
client.get_parameter(Name=arn)      # works
client.get_parameters(Names=[arn])  # {} + the ARN listed in InvalidParameters

The cause is the guard in get_parameters:

if name.split(":")[0] in self._parameters:

For an ARN, name.split(":")[0] is "arn", which is never a stored parameter name, so the entry is skipped before get_parameter (which would have handled the ARN) is ever called.

Change

Extract the prefix-stripping already present in get_parameter into _strip_parameter_arn, and use it in the get_parameters guard as well.

The caller's original string stays the dict key, so the response layer still reports InvalidParameters using exactly what was requested rather than a stripped name — there's a test for that. Name in the returned parameter is the bare name, matching the existing get_parameter-by-ARN behaviour.

Tests

• test_get_parameters_by_arn, parametrized by-name / by-arn, mirroring the existing parametrized test_get_parameter from SSM.Client.get_parameter() by ARN instead of Name fails with ParameterNotFound #7748.
• test_get_parameters_by_arn_reports_unknown_arn_as_invalid — an unresolvable ARN comes back in InvalidParameters verbatim.

Against unpatched master these give 2 failed / 1 passed; the by-name case passes either way. Full tests/test_ssm/ suite: 181 passed. mypy and ruff check clean.

Not covered by this PR

Two other things reported in #9396:

• create_resource_share still rejects SSM parameters — SHAREABLE_RESOURCES in moto/ram/models.py has no "parameter" entry. Adding the string is trivial, but AWS only permits sharing Advanced-tier parameters, and RAM has no visibility into SSM state to enforce that. Happy to do it if you have a preference on how strict it should be.
• The reported missing Tier on put_parameter appears to be a non-issue — Tier is stored and returned in response_object on current master.