fix(gemini-enterprise): align stage-0 tfvars sample and make stage-1 internal subnet lookup configurable

[aghassemlouei]

Description

This pull request addresses variable schema alignment and internal load balancer subnet lookup in the Gemini Enterprise blueprint:

• Fixes [Bug] gemini-stage-0: terraform.tfvars.sample diverges from the variable schema (gcs_data_store_names, create_resource_keys, bq_data_store_configs) #109: Aligns gemini-stage-0/terraform.tfvars.sample with actual variable schemas for gcs_data_store_configs and bq_data_store_configs (as maps of objects with create_dataset), and removes the undeclared create_resource_keys.
• Fixes [Bug] gemini-stage-1: internal load balancer subnet data source uses a hardcoded literal name with no override variable #105: Adds a configurable subnet_name variable in gemini-stage-1 and updates the internal load balancer subnet data source to resolve dynamically across Shared VPC and Greenfield deployments.

Fixes #109
Fixes #105

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Deployment & Compliance Impact

• Applicable Regimes:
  • US Region Restricted (e.g., Access Policy constraint)
  • FedRAMP Moderate
  • FedRAMP High
  • DoD IL4
  • DoD IL5
  • General / All
• NIST 800-53r5 Controls: N/A

Checklist

Code Quality & Reusability

• My code adheres to the Maximize Reusability principle. I have not redefined common elements and have reused existing base configurations and modules where possible.
• I have checked that no existing module or configuration in modules/ or fast/ can be leveraged for this change.
• My code follows the established naming conventions outlined in documentation/naming-convention.md.

Documentation

• I have updated the README.md of the modified module or blueprint.
• I have added/updated documentation for inputs (variables) and outputs.

Security

• My change adheres to GCP security best practices and the principle of least privilege.
• I have ensured compliance with the targeted regime (FedRAMP Moderate, FedRAMP High, IL5, etc.).

Testing

• I have tested my changes locally.
• I have included details of my testing in this PR.

Testing Performed

• Validated gemini-stage-0/terraform.tfvars.sample against variables.tf schemas.
• Validated gemini-stage-1 variable declarations and subnet lookup logic.