Run tests against local server by default

[bverbeken]

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[Copilot]

Pull request overview

This pull request enables tests to run against a local development server by default, while maintaining the ability to run against the staging environment in CI. The hardcoded staging URL is replaced with a configurable environment variable that defaults to localhost.

Changes:

• Made test server URL and system API secret configurable via environment variables with sensible defaults for local development
• Updated CI workflow to explicitly set the staging server URL, preserving existing CI behavior
• Refactored create_client from a static method to an instance method to support dynamic URL configuration

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/seatsioClientTest.py	Replaced hardcoded BASE_URL constant with base_url() method that reads from API_URL env var (defaulting to localhost:9001); made system_api_secret() use env var with default; converted create_client from static to instance method; minor import reordering
.github/workflows/build.yml	Added API_URL environment variable pointing to staging server to maintain existing CI test behavior

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