fix: preserve parallel tool call setting

[harivilasp]

Summary

• parse parallel_tool_calls on /v1/responses requests
• forward the field through RequestPayload::to_upstream_request after gateway tool normalization
• add unit and route-level regression coverage proving parallel_tool_calls: false reaches the mock vLLM request

Closes #127.

Why

The gateway already preserves several Responses API generation/control fields (include, temperature, top_p, max_output_tokens, truncation, metadata) while normalizing tools for vLLM. parallel_tool_calls was missing from the typed request model, so clients that explicitly disabled parallel tool calls had that setting silently dropped on the executor path.

Validation

• cargo test -p agentic-server-core
• cargo test -p agentic-server --test responses_test
• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• cargo fmt -- --check

Focused tests were written first and failed before the implementation because parallel_tool_calls serialized as absent/null instead of false.