PII Remover: Restrict config to entity_types enum with pattern-based defaults

[AkhileshNegi]

Summary

Target issue is ProjectTech4DevAI/kaapi-backend#1250

The PII remover config accepted free-form entity_types strings plus engine knobs (threshold, nlp_engine_type, model_name) via the API. Typos like "AADHAR" were saved silently and only failed (or were ignored) at run time, and the default enabled NER-based types (PERSON, LOCATION, NRP), which use an English-only model that is noisy on Indic names and over-redacts.

Changes:

• New PIIEntityType enum; config entity_types: List[PIIEntityType] rejects unknown values.
• Validation at create time: POST /guardrails/validators/configs/ checks pii_remover payloads against PIIRemoverSafetyValidatorConfig, so bad entity_types or unsupported fields return 422 on save, not on the first guardrail run.
• Default is now pattern/regex-based types only (matches the default list in #1250). PERSON, LOCATION, NRP are opt-in.
• threshold, nlp_engine_type, model_name removed from the API config (extra="forbid" rejects them). They remain PIIRemover constructor params for evals.
• Config uses use_enum_values=True so validated values reach Presidio as plain strings.
• README updated: threshold is only effective in transformers mode (spaCy scores every NER hit 0.85; pattern recognizers aren't thresholded).

Opting in to NER types (list replaces the default, so include the pattern types you still want):

{
  "name": "pii-with-names",
  "type": "pii_remover",
  "stage": "input",
  "on_fail_action": "fix",
  "entity_types": ["PERSON", "LOCATION", "PHONE_NUMBER", "EMAIL_ADDRESS", "IN_AADHAAR"]
}

Checklist

• Ran fastapi run --reload app/main.py or docker compose up in the repository root and test.
• If you've fixed a bug or added code that is tested and has test cases.

Notes

• No backward compatibility needed: PII configs created with this change are the first in use.
• PATCH on a validator config does not accept config fields (extra="forbid"); to change entity_types, delete and recreate the config.
• NRP is not produced in transformers mode (CoNLL label mapping has no NRP label); spaCy mode only.
• Tests: test_pii_remover.py 14/14 pass (run with --noconftest; local test DB missing).

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