[v2] Apply sigv4a_signing_region_set when SigV4A is resolved via auth scheme preference

[jonathan343]

This is a port of boto/botocore#3663 for AWS CLI v2

Overview

Fix sigv4a_signing_region_set being ignored when SigV4A is selected via auth_scheme_preference or explicit signature_version='v4a', causing X-Amz-Region-Set to fall back to the endpoint-derived region (e.g., us-west-2 instead of *).

Problem

SigV4A signing-context setup only ran inside set_operation_specific_signer() when auth_type == 'v4a'. This path isn't reached when SigV4A is resolved later by _set_auth_scheme_preference_signer() or configured directly on the client, so the region set was never applied.

Solution

Extract the SigV4A signing-context update into a shared helper and call it whenever the resolved signer is v4a, preserving existing region-set precedence.

Testing

Added unit coverage for:

• auth_scheme_preference='sigv4a' applying the configured SigV4A region set
• explicit signature_version='v4a' applying the configured SigV4A region set

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[aemous]

LGTM. I've also verified the signing region set config is correctly being used in AWS CLI v2 on this branch.