fix(adapter): handle Anthropic-compat gateways without /v1/ in base URL

[AlexMikhalev]

Problem

The Anthropic adapter's get_service_url used format!("{base_url}messages"), which concatenates the base URL with messages without any slash separator. This works for the default Anthropic base URL (https://api.anthropic.com/v1/) because the trailing slash happens to align with the message path. It produces malformed URLs for Anthropic-compat gateways whose base URL doesn't end in /v1/ — e.g. terraphim-llm-proxy's MiniMax provider at https://api.minimax.io/anthropic was producing https://api.minimax.io/anthropicmessages (404).

This affects both Chat and ChatStream service types in the same way, so streaming requests to MiniMax (and any other non-Anthropic-but-compat provider) fail with 404 today.

Fix

Three-way URL construction:

if base_url ends in "messages"             → pass through unchanged
else if base_url ends in "/v1/" or "/v1"   → append "messages" (legacy Anthropic behavior)
else                                       → append "/v1/messages" with proper slash

Tests

6 new unit tests covering all three shapes + the streaming-vs-chat equivalence. 71/71 lib tests pass.

Context

Discovered while validating terraphim-llm-proxy's MiniMax highspeed offerings on 2026-08-11. PR terraphim/terraphim-llm-proxy#19 fixed the config side; this PR fixes the genai adapter side. With both landed, MiniMax highspeed models route correctly under all conditions:

• Non-streaming via minimax_client::send_request ✅ (working today)
• Streaming via genai Anthropic adapter ✅ (fixed by this PR)

Acceptance criteria

• Anthropic default base URL still produces https://api.anthropic.com/v1/messages
• /v1 suffix without trailing slash works (preserves legacy behavior)
• Bare host gets /v1/messages appended with proper slash
• Bare host with trailing slash gets /v1/messages appended after slash trim
• Fully-formed URL ending in messages passes through unchanged
• Streaming and chat use identical URL logic
• cargo test --lib passes all 71 tests (6 new + 65 existing)

Out of scope

The Anthropic adapter also has a separate bug (terraphim-llm-proxy#18) where tools: [] payloads get forwarded to OpenAI-compat upstreams and rejected with tools[0].type:type cannot be empty. That's a request-shape adapter issue in to_web_request_data, not a URL construction issue, and needs a separate fix.