feat: add tox args support for custom CLI flags

[rnetser]

Problem

Currently the tox configuration only maps branches to test environments:

tox:
  main: all
  dev: testenv1,testenv2

There's no way for users to pass custom CLI flags to tox (e.g., -p for parallel, -x for fail-fast, --override for config overrides).

Proposed Solution

Add a tox-args configuration key that accepts a string of CLI flags to pass to tox:

tox:
  main: all
tox-args: "-p -x"

These args will be appended to the tox command built in runner_handler.py.

Requirements

• Add tox-args to config schema (webhook_server/config/schema.yaml)
• Read tox-args in github_api.py _repo_data_from_config()
• Pass args to tox command in runner_handler.py run_tox()
• Add tests for new functionality
• Update example config

Files to Modify

• webhook_server/config/schema.yaml
• webhook_server/libs/github_api.py
• webhook_server/libs/handlers/runner_handler.py
• webhook_server/tests/ (new or existing test files)
• examples/.github-webhook-server.yaml