`--body -` not treated as stdin (parsed as literal JSON)

## Bug

`--body -` fails with `invalid character '-' in numeric literal` — the `-` is parsed as literal JSON instead of meaning "read the body from stdin". The conventional Unix idiom isn't supported. (A bare pipe to stdin with no `--body` flag does work.)

## Root cause (Speakeasy)

The generated body handling in `flagutil.BuildRequest` and `BuildRequestBody` passes the `--body` flag value straight to `json.Unmarshal` without honoring the `-`=stdin convention.

## Stopgap fix (retire when upstream is fixed)

`.speakeasy/patches/internal/flagutil/metadata.go.patch`: new `resolveDashStdin(cmd, val)` helper reads stdin when `val == "-"`, applied in both the whole-body path (`BuildRequest`) and the per-field body path (`BuildRequestBody`).

**To retire:** once the generated body handling treats a lone `-` as stdin, drop those hunks.

## Refs

- Introduced by #23
- Upstream: file to speakeasy-api/speakeasy (generated body/flag handling).

Start with .speakeasy/patches/internal/flagutil/metadata.go.patch and inspect the referenced flagutil.BuildRequest and BuildRequestBody paths. Reproduce the issue with --body - and stdin input, then verify that a lone dash is read from stdin in both whole-body and per-field handling without changing the existing bare-stdin behavior.

Written by the indexing model from the issue text.