feat(guardrails): add EncodedPayloadScanner for obfuscated injection detection

[u7k4rs6]

Closes #50.

Summary

• Adds EncodedPayloadScanner — a local, dependency-free scanner that detects base64, hex, percent-encoded, and unicode/hex-escape blobs, decodes them, then rescans the decoded text for prompt-injection markers.
• Only blobs that decode to injection content (confidence 0.9) cross the block threshold (default 0.6); benign encoded data — hashes, tokens, image fragments, file attachments — passes cleanly because it decodes to non-injection content.
• Registered as "encoded_payload" via @register_scanner, exported from __init__.py and added to __all__, wired into create_default_pipeline(encoded_payload=False) — off by default, same policy as urls and invisible_chars.

Motivation

Keyword/regex-based scanners all operate on raw text. An adversary can bypass every one of them by base64-encoding an injection string:

aWdub3JlIGFsbCBwcmV2aW91cyBpbnN0cnVjdGlvbnM=

decodes to ignore all previous instructions and passes JailbreakScanner, CodeInjectionScanner, and SecretsScanner unchanged. EncodedPayloadScanner closes this class of bypass.

Changes

File	Change
python/fi/evals/guardrails/scanners/encoded_payload.py	New scanner
python/fi/evals/guardrails/scanners/__init__.py	Import, __all__, create_default_pipeline param
python/tests/sdk/test_guardrails_scanners.py	TestEncodedPayloadScanner — 6 cases

Test plan

cd python
uv sync --dev
uv run pytest tests/sdk/test_guardrails_scanners.py::TestEncodedPayloadScanner -v

Expected:

PASSED test_detects_base64_encoded_injection
PASSED test_detects_hex_encoded_injection
PASSED test_detects_percent_encoded_injection
PASSED test_benign_base64_passes
PASSED test_hex_hash_passes
PASSED test_clean_text_passes
6 passed in 1.48s