Function build_credentials_provider 

async fn build_credentials_provider(
    profile: &Profile,
    secret: Option<&Secret>,
) -> Option<SharedCredentialsProvider>

Build a SharedCredentialsProvider for a profile.

• Manual profiles: use the keychain secret directly.
• AWS-discovered profiles: load the full SDK config for the named profile so the SSO / assume-role / credential-process providers are wired in. A bare ProfileFileCredentialsProvider only understands static aws_access_key_id entries and silently fails on SSO profiles.
• Env profiles: fall through to None — the SDK’s default chain will pick up the env vars on its own.

Returns None when no provider can be constructed.