Add comprehensive test coverage for security-critical URL and token
sanitization functions in config_handlers.go. These functions protect
the setup script endpoint from injection attacks.
TestSanitizeInstallerURL (23 cases): empty/whitespace handling, valid
http/https URLs, fragment stripping, query preservation, control character
rejection, invalid scheme rejection (ftp/file/javascript/data), and
missing host validation.
TestSanitizeSetupAuthToken (19 cases): empty/whitespace handling, valid
hex tokens of various lengths (32-128 chars), mixed case hex, control
character rejection, non-hex character rejection, and length validation.
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