fix(handler): prevent double slash in URLs for root path prefix

[boostvolt]

NormalizePathPrefix("") returns / for empty prefix, but URL construction appends /api/..., resulting in //api/health-checks/complete. This causes unnecessary 301 redirects on every health check.

Now returning "" for empty/root prefix instead of /.

[Copilot]

Pull request overview

This pull request fixes a double slash issue in URLs when using an empty/root path prefix. The problem occurred because NormalizePathPrefix("") returned "/", which when concatenated with /api/... resulted in //api/..., causing unnecessary 301 redirects.

Changes:

• Modified NormalizePathPrefix to return empty string for root paths ("" or "/") instead of "/"
• Updated URL construction and conditional checks throughout the codebase to work with the new normalization behavior
• Fixed a pre-existing bug where NormalizePathPrefix would return without trimming trailing slashes when adding a leading slash

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
handler.go	Updated NormalizePathPrefix function to return empty string for root paths, simplified prefix assignment after normalization, updated middleware condition to check for empty string instead of "/", and fixed a bug in path prefix normalization logic
internal/riveruicmd/riveruicmd.go	Simplified health check URL construction by removing redundant TrimSuffix call since normalized prefix never has trailing slashes

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