feat: Add the flag-overrides capability to the contract test service

[kinyoklion]

Summary

The contract test service now advertises the flag-overrides capability and accepts the overrides object in the SDK configuration, as the test harness service specification describes: filePaths (required), duplicateKeysHandling (fail or ignore), changeDetection (polling or watching), and pollIntervalMs. The block configures FileOverrides.source() on the data system builder. An overrides block without a dataSystem block is rejected, because the override source is an option of the FDv2 data system.

The service also reads its listening port from the PORT environment variable when set, which the Makefile already passes, so several test services can run side by side on one machine. The default stays 8000.

This PR depends on the file-based override source (rlamb/overrides-java-file-override-source) and is based on that branch; retarget it to feat/overrides once that branch merges.

The existing file data source keeps its current behavior; this change does not touch its code paths.

SDK-3246

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Note

Overview
The Java server SDK contract test service now supports the flag-overrides harness capability so cross-SDK tests can drive file-based flag overrides.

Create-instance config accepts an optional overrides block (filePaths, duplicateKeysHandling, changeDetection, pollIntervalMs), mapped to FileOverrides.source() on the FDv2 data system builder. overrides without dataSystem is rejected with a clear error, since overrides are a data-system option only.

The service advertises flag-overrides in its status capabilities. It also honors the PORT environment variable for the HTTP listen port (default 8000), so multiple contract-test services can run on one machine.

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[kinyoklion]

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