fix(rack): fiber-safe context detachment in EventHandler

[rsamoilov]

Summary

Fixes context detachment errors in fiber-based environments.

Problem: When using Rack::Events, the on_finish callback can be invoked from a different fiber than on_start (e.g., when streaming response bodies). This causes OpenTelemetry::Context.detach(token) to fail with:

ERROR -- : OpenTelemetry error: calls to detach should match corresponding calls to attach

Solution: Store the fiber reference alongside the token and span. On detach_context:

• Always finish the span (trace data is never lost)
• Only detach context if running in the same fiber
• Log a debug message when detachment is skipped

Note on fiber pooling: Skipping detach could leave stale context in the original fiber. However, servers that reuse fibers across requests would be fundamentally incompatible with any Ruby feature relying on fiber-locals (e.g., ActiveSupport::CurrentAttributes), so this isn't something we should solve at the instrumentation level.

Changes

• Changed env key from otel.rack.token_and_span to otel.context_info
• Changed storage format from [token, span] to [fiber, token, span]
• Updated detach_context to check fiber identity before detaching
• Applied changes to all three convention variants (stable, old, dup)

Test plan

• Added unit tests for same-fiber detachment
• Added unit tests for cross-fiber detachment (verifies span finishes without error)
• All existing tests pass across all 12 configurations (4 Rack versions × 3 conventions)

[linux-foundation-easycla]

• ✅ login: rsamoilov / name: Roman Samoilov (16aa912)
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