docs: fix custom server test runner

[FanouZeng-TT]

Description

The README's "Testing a Custom Server" example currently invokes uv run pytest with --conformance_input and --fixture_config. Those are absl.flags flags used by the individual conformance test files, not pytest options, so the documented command fails before the tests are collected.

This updates the custom-server example to use the same per-file runner pattern already used by the repository's conformance CI, passing the custom server URL, simulation secret, conformance input, and fixture config to each *_test.py file.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected, including removal of schema files or fields)
• Documentation update

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Is this a Breaking Change or Removal?

N/A. This is a documentation-only update and does not remove schema files or fields.

Breaking Changes / Removal Justification

N/A.

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Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules

Validation:

• bash -n /tmp/conformance-readme-runner.sh
• git diff --check
• uvx pre-commit run --all-files