Nullable scalar fields generate as JSON flags — bare strings and RFC3339 dates rejected

**Behavior:** nullable scalar body fields (spec `anyOf: [string|date-time, null]`) generate as `FlagKindJSON`, so bare values fail flag-parse: `--base-config sweep-cfg-a` → `invalid character 's'`, `--start-date 2027-01-01T00:00:00Z` → `invalid character '-'`. Only JSON-quoted values (`'"…"'`) worked. Affected (verified): `--base-config`, `--default-value-config` (features create/update), `--start-date`/`--cutoff-date` (ramp-schedules create/update), `--scheduled-publish-at` (feature-revisions request-review), `--population-id`, `--entity-id` (nullable variants). The same spec property can bind differently per op (schedule-publish got `FlagKindDateTime`; request-review's nullable variant got JSON).

**Root cause:** the generator maps `OptionalNullable[time.Time]` / nullable strings to `FlagKindJSON` instead of the scalar flag kinds.

**Carried fix:** `.speakeasy/patches/internal/flagutil/metadata.go.patch` (PR #48) — `buildJSONField` re-interprets invalid JSON as a JSON string, so bare scalars parse; object/array targets still type-error as before.

**Retire when:** the generator dispatches nullable scalars to their scalar flag kinds (string/datetime) instead of JSON.

Start with .speakeasy/patches/internal/flagutil/metadata.go.patch and reproduce the listed CLI commands using bare strings and RFC3339 dates. Trace generator handling of OptionalNullable[time.Time] and nullable strings, then verify nullable scalars use string or datetime flags while object and array inputs still reject invalid JSON.

Written by the indexing model from the issue text.