fix: Omit non-symbol context attributes and protect against cyclic values

[kinyoklion]

Summary

Attribute references store every path component as a symbol, so a context attribute with a non-symbol name (for example "street" => ... instead of street:) can never be addressed: it is invisible to evaluation and bucketing, and a private-attribute reference can never redact it. Today the event filter still copies such attributes into analytics event payloads, with one extra defect: a top-level string-keyed attribute is emitted with a null value, because the attribute name comes from raw storage but the value is fetched through get_value, which symbolizes the name and misses the entry.

This PR makes ContextFilter:

• Omit attributes with non-symbol names, at the top level and at any nesting depth (including the whole subtree under a non-symbol key). Omitted attributes are not reported in _meta.redactedAttributes, since no reference removed them. This also removes the top-level null emission described above.
• Log an error the first time a non-symbol name is dropped (once per filter instance, guarded by an AtomicBoolean since the formatter runs on flush workers), so the condition is discoverable without log spam.
• Stop recursing into cyclic values. A self-referential hash in a context attribute previously crashed event serialization with SystemStackError. The filter now tracks the chain of hashes from the attribute root down each branch and omits a value that points back into its own chain. The same object appearing in different branches (a DAG) is still copied everywhere it appears.

Behavior change note: string-keyed attribute data that was previously sent in event payloads (intact when nested, null at top level) is now omitted.

Performance: same-process A/B against main on representative context shapes shows 0.99-1.05x on flat and redact-all paths and ~0.90x on deeply nested contexts (the cycle-protection chain costs one small array per nested hash).

Regression tests cover: top-level and nested omission, subtree omission, no redactedAttributes reporting for omitted names, composition with private-attribute redaction, log-once behavior, the previous SystemStackError on cycles, and DAG preservation. All new tests fail against the previous implementation.