docs: SDK OpenAPI spec no longer documents the 2000-character trait value limit

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Describe the bug

The public SDK OpenAPI spec (sdk/openapi.yaml) advertises trait_value as an unconstrained string, but the API still rejects string trait values longer than 2000 characters.

The limit is enforced at runtime in api/environments/identities/traits/fields.py:27 via TRAIT_STRING_VALUE_MAX_LENGTH = 2000:

Value string is too long. Must be less than 2000 character

The spec previously documented it. trait_value.anyOf included {type: string, maxLength: 2000} — but the constraint was dropped in #6653 (flag-engine v10 upgrade, re-applied in #6741). The trait schema now derives from flag_engine.segments.types.ContextValue (None | int | float | bool | str), which carries no length constraint, so manage.py spectacular emits a plain string.

Steps To Reproduce

1. Open the sdk_v1_post_identities reference (or sdk/openapi.yaml) and check traits[].trait_value, the string variant shows no maxLength.
2. Send a trait value of 2001 characters:

curl -X POST https://edge.api.flagsmith.com/api/v1/identities/ \
  -H 'X-Environment-Key: <client-side key>' \
  -H 'Content-Type: application/json' \
  -d "{\"identifier\": \"repro-user\", \"traits\": [{\"trait_key\": \"long\", \"trait_value\": \"$(printf 'a%.0s' {1..2001})\"}]}"

3. The API responds 400 Bad Request:

{"traits": [{"trait_value": ["Value string is too long. Must be less than 2000 character"]}]}

4. Per the published spec, this request is valid

Expected behavior

TraitInput.trait_value and Trait.trait_value in the spec should document maxLength: 2000 on the string variant, matching what the API enforces.

Screenshots

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