fix(vertex): async client missing us/eu multi-region base_url branches

[atob1]

Summary

AnthropicVertex.__init__ (sync) maps region="us" to https://aiplatform.us.rep.googleapis.com/v1 and region="eu" to the .eu.rep.googleapis.com hostname (added in 0.89.0). Those branches were never added to AsyncAnthropicVertex.__init__, so passing region="us" / "eu" to the async client falls through to the generic fallback and builds {region}-aiplatform.googleapis.com, which is not a valid endpoint and returns 404 from the GCP URL router.

This PR mirrors the two elif branches from the sync class into the async class. 4 lines added, no other changes.

Repro (SDK 0.96.0, before fix)

import anthropic, asyncio

async def main():
    c = anthropic.AsyncAnthropicVertex(project_id="YOUR_PROJECT", region="us")
    print(c.base_url)
    # https://us-aiplatform.googleapis.com/v1/   <-- wrong hostname
    await c.messages.create(
        model="claude-opus-4-7",
        max_tokens=16,
        messages=[{"role": "user", "content": "hi"}],
    )
    # anthropic.NotFoundError: Error code: 404 (HTML page from GCP URL router)

asyncio.run(main())

After fix

c.base_url becomes https://aiplatform.us.rep.googleapis.com/v1/ — matches the sync client and the working US multi-region endpoint, and the request succeeds.

Test plan

• Unit: add a test that AsyncAnthropicVertex(region="us").base_url ends with aiplatform.us.rep.googleapis.com/v1/ (mirror whatever the sync test does)
• Manual: verified messages.create now returns 200 against region="us" with claude-opus-4-7