feat: Support the flag-overrides capability in the contract test service

[kinyoklion]

Summary

Adds the flag-overrides capability to the contract test service and the overrides block of the SDK configuration it accepts, as described in the test harness service specification. The block's filePaths, duplicateKeysHandling (fail or ignore), changeDetection (polling or watching), and pollIntervalMs map onto FileOverrides.Source() options, and the source is attached to the FDv2 data system builder that the dataSystem block configures. The test service supplies a YamlDotNet parser to the source so the harness's YAML document test can run.

The test service also takes an optional port as its first command line argument, defaulting to 8000, so that several test services can run on one machine.

This PR is based on the branch of the file-based override source change, which it depends on.

SDK-3248

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Note

Overview
The .NET server SDK contract test service now advertises the flag-overrides capability and accepts an overrides block on harness SDK configuration. That block maps to FileOverrides.Source() (file paths, duplicate key handling, change detection, poll interval) and is attached to the FDv2 data system builder; overrides without dataSystem is rejected.

The harness wires YamlDotNet as the override file parser so YAML contract tests can run. The test service executable also accepts an optional port as its first CLI argument (default 8000) so multiple instances can run on one machine.

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[kinyoklion]

bugbot review

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