fix(backend): validate incoming plugin run input context against empty payloads

[annukumar123]

This PR addresses an input validation vulnerability across the backend plugin run router inside backend/routes/plugins.py.

The Problem: Previously, if a user submitted an empty string "" or whitespace-only inputs like " ", the text would bypass initial checks. This caused edge-case logic calculations to output erratic statistics (e.g., registering Sentences: 1 on an empty string inside the _wordcount plugin), executing unnecessary subprocess sandboxes, or writing meaningless telemetry logs to the database.

The Fix: Added a robust boundary validation check right at the entry point of the run_plugin route handler. If the stripped input string is empty, the server now halts execution immediately and raises a clean, structured HTTPException(status_code=400, detail="..."). This protects all downstream plugin functions and keeps server resources stable.

Hackathon Association

Program: Social Summer of Code (SSoC) 2026
Please label this PR with the appropriate SSoC difficulty tags (e.g., ssoc, level-1) so it registers for leaderboard points.

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