fix: replace account ID with placeholder in PingFederate sample's aws-targets.json

[cls-aws]

Amazon Bedrock AgentCore Samples Pull Request

Concise description of the PR

Changes to PingFederate sample's aws-targets.json, to replace account ID with placeholder.

User experience

Before: aws-targets.json exposed account ID
After: Uses the placeholder 123456789012. The existing deploy script (deploy_sample.sh) overwrites this file at deploy time with the caller's actual account ID, so there is no functional impact.

Checklist

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• Changes have been tested
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