[v2] fix(params): allow zero values for required numeric fields - #4014

Backport: #4012

Fixes #3866

Links to the line numbers/files in the OpenStack source code that support the
code in this PR:

• https://docs.openstack.org/keystone/latest/admin/unified-limits.html (Keystone permits a default_limit of 0)
• https://docs.openstack.org/api-ref/identity/v3/index.html#create-registered-limit

Problem

registeredlimits.CreateOpts.DefaultLimit is declared as int with
required:"true", and BuildRequestBody rejects the zero value of any
required field. As a result the SDK fails client-side with Missing input for argument [DefaultLimit] when creating a registered limit with a
default_limit of 0, even though Keystone accepts 0 as a valid limit
(valid range is -1 to 2147483647).

Solution

BuildRequestBody, BuildQueryString, and BuildHeaders no longer treat a numeric zero as missing input for
fields tagged required:"true". Zero is legitimate input for numeric
fields, so range validation is left to the server. Other zero values
(empty strings, nil pointers, empty structs, ...) are still rejected
client-side as before, so existing validation behavior is unchanged
everywhere else.

This implements option 1 suggested by @winiciusallan in the issue,
scoped to numeric kinds so the existing ErrMissingInput expectations
for string/struct/pointer fields keep working.

Validation

• Added TestBuildRequestBodyRequiredNumericZero (testing/params_test.go):
  required int zero passes and is serialized; required string empty
  still returns ErrMissingInput.
• Extended TestBuildQueryString and TestBuildHeaders to verify required
  numeric zero values are serialized while other required zero values remain
  rejected.
• Added TestCreateRegisteredLimitWithZeroDefaultLimit plus fixtures
  (openstack/identity/v3/registeredlimits/testing/): end-to-end create
  with DefaultLimit: 0, asserting the request body sent on the wire.
• Verified the new test fails before the fix with exactly the reported
  error (Missing input for argument [DefaultLimit]) and passes after.
• Full unit suite (go test ./...) passes, except the pre-existing
  TestNormalizePathURL failure which also fails on a clean checkout on
  Windows (path/URL normalization, unrelated to this change).
• go vet is clean on the touched packages, including the acceptance
  test package (-tags acceptance).
• Extended the TestRegisteredLimitsCRUD acceptance test with a
  default_limit: 0 create/assert/delete block (not run here, requires a
  live OpenStack cloud).