Move onRequest from prehandler to get and post handlers

[Andrew-Folga]

Proposed change

PoC: Move onRequest logic to request-time handler in DXT plugin

This PR moves the onRequest callback from makeLoadFormPreHandler (pre-handler) to redirectOrMakeHandler (request-time handler), allowing request-specific logic to access the fully-initialised FormContext and page state.

Key changes:

• makeGetHandler and makePostHandler now accept an optional onRequest callback and call it inside redirectOrMakeHandler.
• redirectOrMakeHandler handles onRequest safely, returning any ResponseObject from the callback directly.
• Removed unnecessary type checks and unsafe property access (isTakeover) in favor of proper TypeScript-safe handling.
• Pre-handler (makeLoadFormPreHandler) no longer attempts to invoke onRequest; it only ensures the form model is loaded.

Why:

• Previously, onRequest ran at pre-handler time, so it could not access request-specific page state.
• This refactor moves the logic to the correct lifecycle stage, making it possible to fetch/save state, trigger page-specific HTTP events, and handle redirects safely.

Jira ticket: https://eaflood.atlassian.net/browse/TGC-844

Type of change

• Bug fix
• New feature
• Breaking change
• Misc. (documentation, build updates, etc)

Checklist

• You have executed this code locally and it performs as expected.
• You have added tests to verify your code works.
• You have added code comments and JSDoc, where appropriate.
• There is no commented-out code.
• You have added developer docs in README.md and docs/* (where appropriate, e.g. new features).
• The tests are passing (npm run test).
• The linting checks are passing (npm run lint).
• The code has been formatted (npm run format).

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud