Fix generate_stream error in ClientV2

[fern-support]

Problem

Both the fern-generated Client and ClientV2 assign to self._raw_client. The .fernignore'd wrapper ClientV2 inherits from both, so its _raw_client parameter was getting set to the V2 only (since it is constructed last).

Fix

Create a proxy raw client that will delegate to the v2 and fall back to v1 raw client.

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Note

Low Risk
Small, localized change to client initialization and attribute delegation; primary risk is subtle routing of raw client attributes between v1 and v2.

Overview
Fixes ClientV2/AsyncClientV2 method loss caused by _raw_client being overwritten when initializing both v1 and v2 client bases.

Adds a _CombinedRawClient proxy and updates both constructors to capture the v1 raw client, initialize v2, then replace _raw_client with a delegating wrapper (v2-first, v1 fallback) to keep legacy calls like generate_stream working. Tests add a small assertion that generate and generate_stream remain available on ClientV2.

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