fix: Stop per-context private attributes from leaking between contexts

[kinyoklion]

Requirements

• I have added test coverage for new or changed functionality
• I have followed the repository's pull request submission guidelines
• I have validated my changes against all supported platform versions

Related issues

Found while investigating the contract test failures fixed in #415. Not covered by the current contract test suite.

Describe the solution you've provided

ContextFilter#filter_single_context combined the globally configured private attributes with the context's own _meta.privateAttributes using Array#concat, which mutates @private_attributes in place. Every context filtered by a ContextFilter therefore permanently added its private attributes to the filter's configuration, so a context's private attributes were applied to all contexts filtered afterwards — including the other kinds of the same multi-kind context.

Before (single ContextFilter.new(false, [])):

filter.filter(user_with_private_email)  # {..., _meta: {redactedAttributes: [:email]}}
filter.filter(other_user)               # {..., _meta: {redactedAttributes: [:email]}}  <- email was not private here

The fix builds a new array (@private_attributes + context.private_attributes) instead of mutating the configured list. Event processors reuse a single ContextFilter for the lifetime of the client, so the leak was cumulative across all events.

Describe alternatives you've considered

Constructing the filter per event — unnecessary allocation churn, and the mutation is the actual defect.

Additional context

Full contract test suite against harness v3.2.0-alpha.6 passes with this change (combined with #415): 4723 total, 14 skipped, all ran passed.

Link to Devin session: https://app.devin.ai/sessions/316afaccd2604f8d802a72c9080f6921
Requested by: @kinyoklion

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@cursor review