fix: Replace MD5 with SHA256 for cache key hashing

[Jenisish]

Description

Replaced MD5 hashing with SHA256 for cache key generation in the cache service to address security concerns raised in issue #382

Additionally:

• Added a v2 cache key prefix to make cache invalidation explicit during the hashing migration.
• Added unit tests verifying SHA256-based cache key generation.
• Added a regression test ensuring MD5-based keys are no longer used.

Related Issue

Fixes #382

Type of change

• Bug fix
• New feature / enhancement
• Documentation update
• Test addition
• Refactor

Checklist

• I have read CONTRIBUTING.md
• My branch is up to date with main
• I have run pytest -v and all tests pass
• I have not introduced duplicate issues or features
• My PR title follows the format: feat/fix/docs/test: short description
• I have added tests for new features (Level 2 and 3 issues)
• No hardcoded secrets or API keys in my code
• This PR is linked to a GSSoC 2026 issue

Screenshots (if frontend change)

N/A

Test evidence

pytest -v

[imDarshanGK]

@Jenisish update your branch with the latest main changes

[Jenisish]

Hi @imDarshanGK , I've updated the branch with the latest main changes. Could you please review when you get a chance?

[imDarshanGK]

@Jenisish update the branch with the latest main changes

[Jenisish]

@imDarshanGK updated with the latest main changes.