Migrate session.controller to TypeScript

[dvynshuu]

Issue:
Part of the 2025 pr05 Grant: Incremental Typescript Migration of the p5.js Web Editor

This PR incrementally migrates the backend session.controller.js file to TypeScript (session.controller.ts) and adds full unit test coverage according to the established TS migration guidelines.

Demo:
N/A - Backend / TypeScript migration only.

Changes:
Renamed server/controllers/session.controller.js to server/controllers/session.controller.ts.
Added Types: Typed the Express middleware methods (createSession, getSession, destroySession) using RequestHandler and Express.User.
Added JSDoc Comments: Included comprehensive JSDocs specifying methods, endpoints, and descriptions for all controller methods.
Added Unit Tests: Created server/controllers/tests/session.controller.test.ts with 10 unit tests covering all success/failure paths, utilizing jest-express for mocking the request/response context.
Fixed minor linting (shadowing) warnings that appeared after conversion.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• has no typecheck errors (npm run typecheck)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes text editor + canvas sometimes don't load on Safari #123
• meets the standards outlined in the accessibility guidelines

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