fix: Report slash- and tilde-prefixed redacted attributes as escaped references

[beekld]

Summary

When a context has a custom attribute whose literal name begins with /, the redaction logic that builds an event's _meta.redactedAttributes failed to list it.

The serializer converted each literal attribute name to a Reference via Reference::new(key), which interprets a leading / as attribute-pointer syntax. For an attribute literally named /ssn, that parsed to the pointer component ssn, so the value lookup found nothing and the attribute was silently dropped from redactedAttributes. The value itself was never emitted, so there was no data leak. The attribute was just never reported.

This builds the reference from the literal name using the ATREF backwards-compatibility escaping (~ -> ~0, / -> ~1, prepend /), so /ssn is now reported as /~1ssn.

This fix is needed for the rust-server-sdk to pass the v3 contract tests.