.Net: fix(security): fix improper URL encoding and add validation in Search…

[SajanGhimire1]

…UrlPlugin

Security Issues Fixed:

1. Contextual Encoding Mismatch: Used UrlEncoder.Default (HTML context) instead of Uri.EscapeDataString (URL context) for query parameters
2. Query Parameter Injection: Special characters (&, =, #, ?) not properly encoded
3. Protocol Injection: No validation for javascript:, data:, http:// schemes
4. DoS Risk: No length limits on query parameters

Changes:

• production: Use Uri.EscapeDataString() for URL-safe encoding
• production: Add ValidateQuery() method with security checks
• production: Add GenerateSearchUrl() helper for consistency
• tests: Update all assertions to expect proper encoding
• tests: Add security validation tests

Fixes CWE-138: Improper Neutralization of Special Elements

Motivation and Context

Description

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

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