fix(pii): Validate PII regexes for byte-mode compilation

[sentry]

This PR addresses an issue where PII scrubbing regexes containing Unicode character ranges (e.g., Korean Hangul) would pass initial configuration validation but fail at runtime during byte-level attachment scrubbing.

The root cause was that relay-pii/src/attachments.rs:apply_regex_to_utf8_bytes recompiles regexes with unicode(false) for byte-mode matching, which does not support Unicode character ranges. However, the PII configuration validation in relay-cabi/src/processing.rs:relay_validate_pii_config (via CompiledPiiConfig::force_compile()) only checked for standard (unicode-enabled) regex compilation.

To fix this, the force_compile() method in relay-pii/src/compiledconfig.rs has been enhanced. It now performs an additional validation step for user-defined Pattern and RedactPair regexes: it attempts to compile them using regex::bytes::RegexBuilder::new(...).unicode(false).build(). If this byte-mode compilation fails, a PiiConfigError::RegexError is returned, causing the PII configuration to be rejected at validation time. This prevents invalid PII configurations from being accepted and deployed, thereby eliminating runtime errors.

Additionally, a comment in relay-pii/src/attachments.rs has been updated to reflect this improved validation, acknowledging that byte-mode compilation errors for user-defined patterns should now be caught earlier.

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Fixes RELAY-5Y