fix: add centralized @ControllerAdvice exception handler

[PranavAgarkar07]

Description

Uncaught backend exceptions were leaking raw stack traces and internal details to API users, posing a security risk.

Changes

• Created GlobalExceptionHandler with @RestControllerAdvice
• Added structured JSON error responses (error, message, status, timestamp)
• Covers: validation errors (400), not found (404), access denied (403), bad request (400), generic errors (500)
• Created NotFoundException and ResourceNotFoundException custom exceptions

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Related Issue

Closes #1041

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[github-actions]

Hi @PranavAgarkar07, thanks for contributing to Nyay Setu! 🎉

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[PranavAgarkar07]

@viru0909-dev This PR is ready for review — CI passes, all labels applied (including gssoc:approved). Could you take a look and merge when convenient? Thanks!

[viru0909-dev]

Requires Changes. Please address the following issues:

Backend tests failed:\n\n[ERROR] COMPILATION ERROR : [ERROR] /private/tmp/nyay-setu-working-fix/backend/nyaysetu-backend/src/main/java/com/nyaysetu/backend/exception/GlobalExceptionHandler.java:[45,95] cannot find symbol [ERROR] Failed to execute goal org.apache.maven.plugins:maven-compiler-plugin:3.11.0:compile (default-compile) on project nyaysetu-backend: Compilation failure [ERROR] /private/tmp/nyay-setu-working-fix/backend/nyaysetu-backend/src/main/java/com/nyaysetu/backend/exception/GlobalExceptionHandler.java:[45,95] cannot find symbol [ERROR] symbol: class AuthorizationDeniedException [ERROR] location: package org.springframework.security.authorization [ERROR] [ERROR] -> [Help 1] [ERROR] [ERROR] To see the full stack trace of the errors, re-run Maven with the -e switch.\n

[viru0909-dev]

Review Summary

This PR has been reviewed and requires changes before it can be approved. The issues below must be resolved.

Issues to Resolve

Result	Check	Notes
FAIL	Description	Description is missing: a Type of Change section; a Checklist section with checkboxes.
FAIL	Checklist	No checklist found in the description.

Passing Checks

Result	Check	Notes
PASS	Title	Title format is acceptable.
PASS	Issue Link	Issue is linked with a closing keyword.
PASS	Merge Conflicts	Conflict status is not yet computed by GitHub — skipping.
PASS	Branch Freshness	Branch was last updated 2.5 days ago.
PASS	CI / Tests	No CI checks are configured or results are not yet available.
PASS	Code Quality	No code quality issues detected in the diff.

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To proceed: push the required fixes to fix/1041-global-exception-handler and the PR will be re-evaluated on the next review run.