fix(anthropic): strip trailing /v1 from base_url in build_anthropic_client (#24833)

[rodboev]

Summary

The Anthropic SDK appends /v1 to base_url when constructing request URLs. When a user configures model.base_url or ANTHROPIC_BASE_URL with a trailing /v1 (the value the older setup guidance suggested), the SDK produces /v1/v1/messages and every request 404s.

The bearer-hook constructor _build_anthropic_client_with_bearer_hook already strips a trailing /v1 at line 612, but its sibling build_anthropic_client — the path used for OAuth, x-api-key, and plain Bearer auth (the common case) — forwarded the value verbatim. This applies the same strip in the main constructor, aligning the two paths.

Fixes #24833

Changes

• agent/anthropic_adapter.py: strip trailing /v1 or /v1/ from normalized_base_url in build_anthropic_client(), immediately after _normalize_base_url_text(), mirroring the existing idiom in the bearer-hook constructor (+4 lines)
• tests/agent/test_anthropic_adapter.py: two new tests covering /v1 and /v1/ suffix stripping (+13 lines)

Validation

Scenario	Before	After
base_url: https://api.anthropic.com/v1	SDK builds /v1/v1/messages → 404	stripped to https://api.anthropic.com → correct
base_url: https://proxy.example.com/anthropic/v1/	/v1/v1/messages → 404	stripped to https://proxy.example.com/anthropic → correct
base_url: https://custom.api.com (no /v1)	unchanged	unchanged
Azure endpoint (*.azure.com/models/anthropic)	unchanged (uses api-version query)	unchanged
Azure Foundry endpoint	unchanged (bearer-hook path)	unchanged
No base_url set	default Anthropic endpoint	unchanged

Test plan

• pytest tests/agent/test_anthropic_adapter.py -v --timeout=0 — 156 passed, 1 skipped
• test_custom_base_url passes (non-/v1 URLs untouched)
• test_azure_anthropic_endpoint_keeps_context_1m_beta passes (Azure unaffected)
• test_azure_foundry_anthropic_endpoint_uses_bearer_auth passes (Foundry unaffected)
• Manual: model.base_url: https://api.anthropic.com/v1 no longer 404s

[rodboev]

Withdrawing this in favor of #45842, which already landed with the overlapping Anthropic base URL fix. Closing to keep the queue clean.