Propagate OpenTelemetry context to async thread pool workers

[KirIgor]

No description provided.

[tycooon]

Reviewed the OTel propagation and the Ruby 3.3 modernization sweep.

The core logic is sound: yield inside the wrapper block correctly reaches the caller's block on the worker thread (and avoids the Ruby 3.3 SyntaxError on anonymous forwarding inside block bodies), attach/detach are correctly paired via ensure on whichever thread executes — including PreemptableProxy's preemption path and its exception rethrow — every hash-shorthand site resolves to a real local/kwarg/let, the checked_transaction/migrator/upsert call sites still match, and keeping opentelemetry-api out of the gemspec is the right dependency design.

Inline comments below: versioning of the release, the shape of the defined?(OpenTelemetry) gate, two test-coverage gaps, and a few simplifications.

One non-blocking note: apps that enable the standard opentelemetry-instrumentation-concurrent_ruby (it patches ThreadPoolExecutor#post with the same with_current wrapping) will get harmless double propagation on top of this. A short README note about the new behavior would cover that and general discoverability.

[tycooon]

Re-review of the current head (b55d1ee) against origin/master, focused on what changed since my 2026-07-03 review — the #47 refactor. release.yml is excluded from this review per request.

All eight findings from the previous review are addressed in the code:

• gemspec → 0.19.0 with required_ruby_version >= 3.3 (matches the 3.3/3.4 CI matrix)
• gate is now defined?(OpenTelemetry::Context) — no more NameError when only a partial/stub OTel is loaded (I confirmed defined? returns nil rather than raising for a non-module OpenTelemetry on 3.3/3.4)
• ROOT short-circuit added, so an app with opentelemetry-api loaded but no active span keeps the zero-overhead plain path
• attach/detach delegated to OpenTelemetry::Context.with_current
• plain/non-OTel branch is now covered (the stub_const "defined without Context" example genuinely hits the else), and the require "opentelemetry-api" is isolated to its own spec file
• detach/cleanup is covered — "restores context after block completes" on a single-thread executor, plus the raise case
• the no-op detach(attach(ROOT)) hook is gone
• upsert YARD now documents target: with a corrected example

I verified locally that the anonymous-&-inside-a-block form (with_current(otel_context, &)) both parses and forwards the caller's block correctly at runtime on Ruby 3.3 and 3.4 — my earlier worry about a 3.3 SyntaxError there was unfounded. CI is green on both.

One optional simplification inline. Nothing blocking on the code I reviewed.