Function extractSearchParams

Walk the supplied search params, validate each one against its requirement (if any), and produce a record suitable for both URL building (client) and request validation (server).

The form of each output value matches its requirement:

• Shape whose runtimeType is a single string: output is a single string.
• Shape whose runtimeType is string[]: output is an array of strings.
• Shape with a union runtime type: output preserves the form that satisfies the shape.
• RegExp: output is a single string.
• RegExp[]: output is an array of strings. Each value must match at least one of the supplied regexes.
• No requirement: output preserves the supplied form (single or array), stringified.

Non-string primitives are coerced with String(...) so that URL builders can accept numbers and booleans even when the shape is string-typed.