feat: run test suites with YAML test files and config-driven gateway probing

[SteveLauC]

The test runner only looked for loadgen.dsl and silently fell back to paths relative to the loadgen config, while env variables defined in a per-test loadgen.yml were never substituted because template rendering used the top-level config's global environment, leaving request URLs pointing at "not_found" hosts. Gateway startup probing was equally fragile: it dialed two ports from LR_GATEWAY_HOST/LR_GATEWAY_API_HOST and treated the gateway as ready once either answered, which raced with multi-entry configurations and with the API listener coming up before the entry.

The runner now accepts either loadgen.dsl or loadgen.yml per test case (DSL preferred) and renders $[[env.X]] against merged environments, following the framework precedence OS env > per-test env > top-level env without mutating the global app config. A gateway.yml remains optional; when present, its listening addresses (the api listener plus every enabled entry) are derived by parsing it with the framework config loader so templates resolve exactly as the gateway binary resolves them, and the gateway is considered ready only once all of them accept connections. The gateway subprocess runs with the test directory as its working directory, startup failures include the captured gateway output, aborted tests surface their reason in the result summary, and request logging prints the final URL after template substitution instead of the raw template.

Standards checklist

• The PR title is descriptive
• The commit messages are semantic
• Necessary tests are added
• Updated the release notes
• Necessary documents have been added if this is a new feature
• Performance tests checked, no obvious performance degradation