BFCL-v4 multi-turn runner: reuse HttpClientSampleIssuer instead of bespoke HTTP client

Follow-up to #346 (review thread on src/inference_endpoint/evaluation/bfcl_v4_multi_turn_runner.py).

In #346, @arekay-nv and @viraatc noted that the BFCL-v4 multi-turn runner reimplements its own HTTP client and response parser instead of reusing the existing sample-issuer path.

Scope

• Drive the multi-turn conversation loop through HTTPEndpointClient / HttpClientSampleIssuer, reusing the existing adapters and event logging.
• Keep only BFCL-specific turn orchestration (custom tool execution between turns) in the runner.
• Drop the bespoke httpx client and response parser from the runner.

Validation

• Diff multi-turn accuracy scores before/after against a live server to confirm parity.

Notes

• No functional change to the single-turn submission path; this is a refactor and does not block #346.