chore: add TypeScript `Strict Mode` Support for the packages

### Current Limitation

Currently, TypeScript strict mode is not consistently enabled across all packages in the monorepo. This leads to:

- Potential runtime errors that could be caught at compile time
- Implicit `any` types that reduce type safety
- Missing null/undefined checks that could cause runtime issues
- Inconsistent type checking standards across different packages
- Reduced developer experience due to less precise type inference

Without strict mode enabled, the codebase may contain:
- Functions with implicit return type `any`
- Variables that could be `null` or `undefined` without proper checks
- Less strict function parameter checking
- Weaker type checking for object property access

### Suggested Improvement

Enable TypeScript strict mode across all packages and fix related type issues:

## Implementation Plan

> [!NOTE]
> Start with the following order:
> 1. @asgardeo/javascript
> 2. @asgardeo/browser
> 3. @asgardeo/react
> 4. @asgardeo/node
> 5. @asgardeo/nextjs

1. **Enable strict mode in tsconfig.json files**
- Add `"strict": true` to all package-level `tsconfig.json` files
- Ensure consistent strict mode configuration across the monorepo

2. **Fix type errors systematically**
- Address implicit `any` types by adding proper type annotations
- Add null/undefined checks where needed using optional chaining and nullish coalescing
- Fix function return type annotations
- Resolve strict property initialization issues

3. **Update build process**
- Ensure CI/CD pipeline fails on strict mode violations
- Update lint rules to complement strict mode checks

4. **Documentation updates**
- Update contributing guidelines to mention strict mode requirements
- Add examples of proper typing patterns

## Files to be updated
- `packages/*/tsconfig.json` - Enable strict mode
- Various `*.ts` and `*.tsx` files - Fix type issues
- CI configuration - Ensure strict checking in build pipeline]

### Please select the package issue is related to

@asgardeo/express, @asgardeo/browser, @asgardeo/javascript, @asgardeo/nextjs, @asgardeo/node, @asgardeo/nuxt, @asgardeo/react, @asgardeo/react-router, @asgardeo/vue

### Version

current

### Reporter Checklist

- [x] I have searched the existing issues and this is not a duplicate.
- [x] I have provided all the necessary information.
- [x] I have verified the improvement is not available in the latest version of the package.

Beginne mit der paketweiten tsconfig.json für @asgardeo/javascript, arbeite dann die aufgeführte Paket-Reihenfolge ab und untersuche die betroffenen *.ts- und *.tsx-Dateien. Aktiviere den Strict-Modus, behebe die daraus resultierenden Typfehler, aktualisiere die CI- und Contributing-Dokumentation und verifiziere, dass alle Pakete die Strict-Prüfung bestehen.

Vom Indexierungsmodell aus dem Issue-Text verfasst.