Toxic Exposure: Add basic toxicExposure node to pre-submission JSON

[sethdarragile6]

This PR adds the top-level toxicExposure JSON node to the pre-submission JSON. At this stage, this is merely a pass-through. Later, it will be transformed by the Form526ToLighthouseTransformer into the Lighthouse request format

Summary

• This work is behind a feature toggle (flipper): YES (disability_526_toxic_exposure)
• Disability Benefits Experience - Team 1 (DBEX-TREX🦖)

Related issue(s)

• Toxic Exposure - Data Transformation Status endpoint #2 - Save TE fields for future transformation#79935

Testing done

• New code is covered by unit tests
• Describe what the old behavior was prior to the change
  Pre-submission form json minus the new toxicExposure node would NOT be added to the submission form json
• Describe the steps required to verify your changes are working as expected. Exclusively stating 'Specs run' is NOT acceptable as appropriate testing
  With the disability_526_toxic_exposure flag ON, submit a 526ez with TE questions answered, confirm that the TE nodes make it to the saved submission record
• If this work is behind a flipper:
  • Tests need to be written for both the flipper on and flipper off scenarios. Docs.
  • What is the testing plan for rolling out the feature?

Screenshots

Note: Optional

What areas of the site does it impact?

(Describe what parts of the site are impacted andifcode touched other areas)

Acceptance criteria

• I fixed|updated|added unit tests and integration tests for each feature (if applicable).
• No error nor warning in the console.
• Events are being sent to the appropriate logging solution
• Documentation has been updated (link to documentation)
• No sensitive information (i.e. PII/credentials/internal URLs/etc.) is captured in logging, hardcoded, or specs
• Feature/bug has a monitor built into Datadog or Grafana (if applicable)
• If app impacted requires authentication, did you login to a local build and verify all authenticated routes work as expected
• I added a screenshot of the developed feature

Requested Feedback

(OPTIONAL)What should the reviewers know in addition to the above. Is there anything specific you wish the reviewer to assist with. Do you have any concerns with this PR, why?