feat: add deterministic test provider for staging/testnet closes #459

[kimanicode]

Closes #459

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Summary

Adds a TEST_PROVIDER_MODE environment variable that activates a fixture-driven test provider, enabling the AI service to run without any API keys in staging/testnet/CI environments. The test provider returns stable, deterministic responses loaded from JSON fixture files, selected via SHA-256 hash of the request content — guaranteeing the same input always produces identical output.

Changes

New files:

• services/test_provider.py — TestProvider class that loads endpoint-specific fixtures from fixtures/ and returns deterministic responses based on a hash of the serialized request data
• fixtures/humanitarian_responses.json — 6 fixture responses covering credible, inconclusive, and not_credible verdicts with varied confidence scores

[Cedarich]

Kindly resolve conflict

[Cedarich]

@kimanicode

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