fix(demos): update JSONPath for Hurl 8

[SomSamantray]

Summary

The currency exchange and weather forecast demo assertions remain valid with Hurl 8.0.0, which rejects dotted JSONPath access to the hyphenated x-arch-state member.

Fixes #925.

Validation

• Both Hurl fixtures passed against a local stub server with Hurl 8.0.0 and the repository-pinned Hurl 4.0.0.
• uvx pre-commit run --files ... passed, and a scan of tracked .hurl files found no remaining dotted access to hyphenated JSONPath members.
• The full Docker-backed demo runner could not run because Docker and planoai are unavailable in this environment.

AI disclosure: OpenAI Codex assisted with issue analysis, implementation, and verification.