fix(detectors): reject null invoice threshold

[DevaanshPathak]

Summary

• reject an explicitly configured max_invoice_amount=None during detector initialization
• preserve the existing default-threshold behavior when the configuration key is omitted
• add focused regression coverage for null, omitted, and valid positive values

Root cause

The validation guard checked whether the retrieved value was non-null rather than whether the configuration key was present. This made an explicit null value indistinguishable from an omitted setting during validation, even though later lookup returned the null value instead of the default threshold.

Impact

Invalid configuration now fails fast with a clear ValueError instead of allowing the detector to initialize and later crash with a TypeError while processing an approval event.

Testing

• uv run pytest tests/unit/ctf/test_invoice_threshold_bypass_config.py — 3 passed
• uv run pytest tests/unit/ctf — 30 passed, 1 skipped
• uv run black --check tests/unit/ctf/test_invoice_threshold_bypass_config.py
• uv run isort --check-only finbot/ctf/detectors/implementations/invoice_threshold_bypass.py tests/unit/ctf/test_invoice_threshold_bypass_config.py

Fixes #125

[Copilot]

Pull request overview

This PR fixes configuration validation for InvoiceThresholdBypassDetector so that an explicitly configured max_invoice_amount=None is rejected during initialization, while leaving the existing “omitted key uses default threshold” behavior intact.

Changes:

• Update _validate_config() to validate max_invoice_amount based on key presence (so explicit None is rejected).
• Add unit tests covering explicit None, omitted key, and a valid positive threshold value.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
finbot/ctf/detectors/implementations/invoice_threshold_bypass.py	Adjusts config validation to fail fast when max_invoice_amount is explicitly set to None.
tests/unit/ctf/test_invoice_threshold_bypass_config.py	Adds regression tests for null, omitted, and positive max_invoice_amount configurations.

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