feat: Add flag overrides to the contract test service

[kinyoklion]

Summary

This PR is stacked on #453 because the test service configures the file-based override source that PR adds.

The contract test service now advertises the flag-overrides capability and accepts the overrides block of the SDK configuration described in the test harness service specification. filePaths, duplicateKeysHandling, changeDetection, and pollIntervalMs map to the options of LaunchDarkly::Integrations::FileData.override_source, which is set on the FDv2 data system builder. An overrides block without a dataSystem block is an error, as in the Go reference service.

The listen gem joins the contract test bundle so that the harness's watching change detection tests can run. The service port can be set with the PORT environment variable and still defaults to 9000, so the existing Makefile and CI steps are unchanged.

Verification against the prebuilt harness (sdk-test-harness branch rlamb/sdk-2657/flag-overrides-tests at 9ad082c, flag-overrides capability): -run 'flag overrides' ran 38 tests with 0 failures, and the full suite with no filter ran 4733 tests with 0 failures (13 skipped by pre-existing capability and long-running gates). The first run of the watching mode tests failed because the watcher relied on listen, which could not scan the system temporary directory where the harness writes its files; the watcher change in #449 corrects that, and both runs above were made with it in place.

The existing FDv1 and FDv2 file data sources keep their current behavior. This series does not change them; the override feature is additive.

SDK-3249

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Note

Overview
The contract test harness service now supports flag overrides for FDv2 clients: it advertises the flag-overrides capability and maps an overrides config block (filePaths, duplicateKeysHandling, changeDetection, pollIntervalMs) onto LaunchDarkly::Integrations::FileData.override_source via data_system.overrides. Supplying overrides without dataSystem raises an error, matching the reference harness behavior.

The bundle adds the listen gem so watching-based change detection in override tests can run. The Sinatra service PORT can be set via ENV['PORT'] while still defaulting to 9000.

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[kinyoklion]

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