fix(core): fix buildUrlParams array handling and replace deprecated substr

[maniktyagi04]

Description

Fixes #8270

This PR fixes two bugs in the buildUrlParams function:

1. Missing assignment: Array parameter values had a trailing & that wasn't removed
2. Deprecated method: Replaced substr(1) with substring(1)

Changes

• Line 336: Assign the slice result to remove trailing & from array parameters
• Line 346: Replace deprecated substr() with substring()
• Line 321: Export buildUrlParams for testing
• Added test cases to verify the fixes

Testing

• ✅ All existing tests pass (53 tests in core, 41 in CLI)
• ✅ New test cases verify array parameter formatting and initial ampersand removal

Impact

• Type: Bug fix
• Breaking: No
• Platforms: All (affects HTTP plugin URL parameter building)

Before

buildUrlParams({ tags: ['javascript', 'typescript'], key: 'value' })
// Returns: "tags=javascript&tags=typescript&&key=value" ❌
//                                            ^^ double ampersand

After

buildUrlParams({ tags: ['javascript', 'typescript'], key: 'value' })
// Returns: "tags=javascript&tags=typescript&key=value" ✅

Additional Notes

• The substr() method is deprecated and should be replaced with substring() or slice()
• Both fixes are backward compatible and don't change the API