More SSG cleanup for multi-sector

[randywoods]

This pull request updates the handling of SSG (Sector-Specific Guidance) models in both the backend and frontend, replacing the previous use of sector IDs with model IDs throughout the codebase. It also improves the retrieval and propagation of demographic data, ensures consistency in the data model, and includes minor UI and documentation tweaks.

Backend: SSG Model Handling and Demographics

• Replaced all usage of SsgSectorIds with SsgModelIds in the Demographics model, related business logic, and tests, ensuring that SSG models are referenced by model ID instead of sector ID. [1] [2] [3] [4] [5]
• Refactored DetermineSsgModels in CpgBusiness to use a mapping from sector to model ID and return a unique list of model IDs, improving clarity and correctness.
• The demographics POST endpoint now returns the full demographics object after saving, not just the assessment ID.
• Fixed a bug in CpgStructure so that TechDomain is retrieved for the correct assessment.

Frontend: Demographics and SSG Model Propagation

• Updated all relevant Angular components to use and propagate ssgModelIds instead of ssgSectorIds, ensuring the assessment state reflects the correct SSG models. This includes AssessmentDemographicsComponent, AssessmentConfigIodComponent, AssessmentDetailComponent, CsiServiceDemographicsComponent, and CpgPracticesComponent. [1] [2] [3] [4] [5]
• Removed obsolete or renamed SSG sector change handlers and updated documentation/comments for clarity in Angular components. [1] [2]

Other Improvements

• Added the IsAnswerable property and improved answer handling in maturity structure helpers for more accurate question rendering. [1] [2] [3]
• Improved spacing and markup in the CPG summary report for better readability. [1] [2] [3]
• Minor code cleanup and improved documentation in several frontend components. [1] [2] [3] [4] [5]

These changes collectively modernize the SSG model handling and provide a more robust, maintainable demographic data flow throughout the application.

🗣 Description

💭 Motivation and context

🧪 Testing

✅ Pre-approval checklist

• This PR has an informative and human-readable title.
• Changes are limited to a single goal - eschew scope creep!
• All future TODOs are captured in issues, which are referenced in code comments.
• All relevant type-of-change labels have been added.
• I have read the CONTRIBUTING document.
• These code changes follow cisagov code standards.
• All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• Tests have been added and/or modified to cover the changes in this PR.
• All new and existing tests pass.
• Bump major, minor, patch, pre-release, and/or build versions as appropriate via the bump_version script if this repository is versioned and the changes in this PR warrant a version bump.
• Create a pre-release (necessary if and only if the pre-release version was bumped).

✅ Pre-merge checklist

• Revert dependencies to default branches.
• Finalize version.

✅ Post-merge checklist

• Create a release (necessary if and only if the version was bumped).