fix(processing): compile filter regex at init and raise SigmaConfigurationError on invalid pattern

[Romil2112]

Closes #509.

Summary

The filter field on ExternalSourceBaseTransformation was recompiled on every _parse_plaintext and _parse_csv call. A malformed pattern raised a bare re.error at parse time rather than SigmaConfigurationError at pipeline-load time — inconsistent with every other regex field in pySigma and breaking callers that catch SigmaError.

This PR adds a _filter_pattern private field, compiles the regex once in __post_init__ (raising SigmaConfigurationError on bad input), and reuses it in both parse methods. The change mirrors the existing reg_filter_output design exactly.

Changes

• sigma/processing/transformations/external.py

  • New _filter_pattern: re.Pattern[str] | None dataclass field
  • __post_init__: compile filter eagerly; raise SigmaConfigurationError on bad regex
  • _parse_plaintext, _parse_csv: use _filter_pattern instead of re.compile(self.filter)

• tests/test_external_placeholder_transformations.py

  • test_filter_invalid_regex_raises_at_init — bad pattern → SigmaConfigurationError at construction
  • test_filter_pattern_compiled_at_init — valid pattern → _filter_pattern set and correct

All 59 existing external-placeholder tests continue to pass.