Adds support for the SIP REFER BXML verb to the python-sdk, mirroring
the <Transfer> pattern but with the SIP-protocol-mandated success
semantics: a successful REFER terminates the call on Bandwidth's side.

Scope: BXML verb + unit tests only. The referCompleteCallback model
will be regenerated from the OpenAPI spec once api-specs#2142 merges.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Unblocked by api-specs#2142 merging. Hand-written to match the
OpenAPI generator output pattern (mirrors TransferCompleteCallback).

Fields: standard call fields + referCallStatus, referSipResponseCode,
notifySipResponseCode. Excludes cause/errorMessage/errorId (not in spec)
and enqueuedTime (inbound SIP URI calls only).

Adds scenario tests for all four referComplete outcomes:
- Success (202 REFER, 200 NOTIFY)
- Failure: REFER rejected (e.g. 405, no NOTIFY)
- Failure: destination unreachable (202 REFER, 4xx NOTIFY)
- Failure: NOTIFY timeout (202 REFER, no NOTIFY)

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

The Transfer SipUri carries ~12 Transfer-specific params (transfer_answer_url,
uui, auth, fallback URLs) that are invalid in a REFER context. A user passing
any of those would silently produce malformed BXML.

ReferSipUri accepts only uri — matching the spec and the C# SDK's nested
Refer.SipUri class design.

Co-Authored-By: Claude Sonnet 4.6 (1M context) <noreply@anthropic.com>

Remove ReferCompleteCallback (will land later via a separate api-specs-driven
PR once VAPI-3440 merges) and delete the distinct ReferSipUri type. Refer now
takes the existing Transfer-flavored SipUri directly, matching the plain,
validation-free style of the other BXML verb models in this SDK.

Make sip_uri optional (default None) to match how other verbs handle
their nested verbs, guard nested_verbs construction against None, and
add a set_sip_uri() setter so callers can attach the SipUri after
construction (mirrors the ruby/node Refer pattern). Replaced
test_minimal/test_shares_sip_uri_with_transfer with a test covering
the new setter.

…API-3161-add-refer-verb

* Generate SDK with OpenAPI Generator Version

* unit tests

---------

Co-authored-by: DX-Bandwidth <dx@bandwidth.com>
Co-authored-by: ckoegel <ckoegel1006@gmail.com>

VAPI-3161: Add <Refer> BXML verb support

* SWI-12283 Allow Setting Custom Token URL

* update unit tests to not hit live oauth url