VAPI-3437 Add <Refer> BXML verb

[stampercasey]

Summary

• Adds the <Refer> BXML verb, which sends a call off to a SIP endpoint via SIP REFER.
• Reuses the existing Transfer-flavored SipUri type for <Refer>'s SIP URI child element, rather than introducing a separate ReferSipUri type — see VAPI-3437 for the design rationale.
• Refer.SipUriElement's setter validates that only Uri is set (and that it starts with sip:); attaching a SipUri with any Transfer-only attribute (Username, TransferAnswerUrl, Uui, etc.) throws ArgumentException naming the offending attribute(s).
• This PR intentionally does not include the ReferCompleteCallback model — that will land separately once the api-specs schema (VAPI-3440) is merged, so the SDK-generation workflow can produce it consistently across SDKs.

Supersedes #190, which bundled the verb with the callback model and used a nested Refer.SipUri class that unintentionally shadowed the shared SipUri type.

Test plan

• dotnet test passes for TestRefer.cs, including new cases covering each Transfer-only attribute being rejected (both via WithSipUri() and direct SipUriElement assignment) and a regression check that Transfer's own use of SipUri is unaffected
• Docs (docs/Refer.md) reviewed for the Transfer-vs-Refer attribute table

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[ckoegel]

this seems to be trying to replicate the autogenerated model docs for the opeanpi schemas. Since it wont come from the api spec, it will need to be hand maintained going forward, and it is also the only BXML verb to have one of these docs. I'd recommend deleting this, customers can use the guide on the dev docs and see the code snippets we'll add there instead

[ckoegel]

This model got a bit over-engineered. I'm gonna make some changes on the branch to make it more closely align to our other verbs, but for reference:

• we typically leave validation out of BXML models for simplicity, and if validation is needed it can be done on the PV side
• no other verbs only have the WithFieldName methods, these are unnecessary for constructing the verb and add bloat to the model