fix: model deploy pipeline fails when PRE_PROD/PROD env vars are empty

[weklund]

This fixes two bugs in the model deployment seed code:

1. Pipeline fails with "Unable to parse environment specification aws:///" when PRE_PROD_ACCOUNT_ID, PRE_PROD_REGION, PROD_ACCOUNT_ID, or PROD_REGION are not provided. Now these values fall back to DEV environment values, allowing single-account deployments.

2. EventBridge rule name exceeds 64 character limit for long project names. Now truncates project name to fit within the limit.

Also adds unit tests for the constants module to prevent regression.

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Checklist before requesting a review

• I updated CHANGELOG.MD with a description of my changes
• If the change was to a module, I ran the code validation script (scripts/validate.sh)
• If the change was to a module, I have added thorough tests
• If the change was to a module, I have added/updated the module's README.md
• If a module was added, I added a reference to the module to the repository's README.md
• I verified that my code deploys successfully using seedfarmer apply

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.