Serialise list-typed query parameters as repeated key=value pairs

[Smaug123]

List- and array-typed [<Query>] parameters were stringified with .ToString(), producing F# list syntax in the URL — e.g. Gitea's NotifyGetList sent status-types=%5Bx%3B%20y%5D. An int[] query parameter didn't even compile under net9.0 nullness checking (ToString() on the array is string | null).

Each element now contributes its own key=value pair, individually escaped — the "multi" collection format, which is what every array query parameter in the Gitea spec declares, and RestEase's convention for enumerable query parameters. (csv/ssv/tsv/pipes collection formats remain unsupported; nothing consumes them here today.)

Mechanically, the query string becomes a runtime computation: each parameter contributes a string list component ([ "limit=10" ] for scalars, List.map/Seq.map for lists and arrays), and the concatenation is appended to the URL — with separator — only when nonempty. So an empty list contributes nothing, and a query consisting solely of empty lists leaves the URL bare. Required scalar parameters produce byte-identical URLs to before.

Tests were written first and observed failing: TestHttpClient/TestListQueryParam.fs covers repeated pairs, empty-list omission, bare-URL behaviour, per-element escaping, and the array case, driven through a generated client against a mock handler.

Note: this deliberately branches off main rather than stacking on #540, but both rewrite the same query-string codegen block in HttpClientGenerator.fs, so whichever merges second will conflict there. The reconciliation is to unify the component type: #540 builds string option components (Option.map/List.choose) for optional scalars; this PR builds string list components. The union is string list components throughout, with optional scalars mapped via Option.toList.

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