feat: support {:form,data} tagged body tuples

[tsitsiridakisiosif]

No description provided.

[cursor]

PR Summary

Low Risk
Small, targeted middleware encoding rules with tests; no auth or data-layer changes, and existing untagged body behavior is preserved.

Overview
Adds an explicit {:form_urlencoded, data} request body tag so callers can force URL-encoded encoding when a client stacks JSON and FormUrlencoded middleware (where a bare map would otherwise be JSON-encoded).

FormUrlencoded now treats tagged bodies as encodable, unwraps the inner data for encoding (including existing :brackets / custom encode options), and sets application/x-www-form-urlencoded. JSON skips tagged bodies entirely—no JSON encoding and no application/json header—leaving them for the form middleware. Untagged map bodies behave as before.

Tests cover default and bracket encoding for tagged tuples, JSON pass-through, and backward compatibility for plain maps.

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[yordis]

should the name be :form_urlencoded?