test(sdk): add processor test suite coverage for shutdown and timeout scenarios

[bryantbiggs]

Summary

Addresses test coverage gaps identified in #3381:

• Shutdown regression tests: Added shutdown() tests with TokioSpawn*Exporter mocks for all three processors (BatchSpanProcessor, BatchLogProcessor, PeriodicReader) on multi_thread(worker_threads = 1) runtime — verifying the BlockingStrategy fix for force_flush() deadlocks (OTLP MetricExporter deadlock issue #2802) also works for shutdown(), which goes through the same export code path.

• Timeout behavior tests: Added HangingExporter mocks for BatchSpanProcessor and BatchLogProcessor that block forever in export(), verifying shutdown_with_timeout() returns Err(Timeout) as expected. PeriodicReader timeout test is omitted (documented why) due to its hardcoded 5-second timeout making it too slow for the regular test suite.

• SimpleLogProcessor documentation: Improved comments on the two #[ignore]d async exporter deadlock tests, explaining they demonstrate inherent design limitations of SimpleLogProcessor (not bugs), and linking to Processor test suite gaps: no coverage for tokio-dependent exporters or runtime deadlock scenarios #3381 and OTLP MetricExporter deadlock issue #2802. These deadlocks cannot be fixed without changing SimpleLogProcessor's synchronous block_on design — users should use BatchLogProcessor for production async exporters.

• current_thread limitation documentation: Documented across all three processor test files that current_thread runtime with tokio-dependent exporters is a fundamental limitation, while multi_thread(1) (the 1-vCPU k8s pod scenario) is supported via BlockingStrategy.

Design notes

Why shutdown needs separate tests from force_flush: Both shutdown() and force_flush() funnel through the same export function (get_spans_and_export / get_logs_and_export / collect_and_export) which uses BlockingStrategy to properly enter the tokio runtime context. The shutdown path then additionally calls exporter.shutdown(). Without separate tests, a regression in the shutdown-specific code path could go undetected.

Why pending() instead of Notify: The hanging exporter mocks use futures_util::future::pending() rather than tokio::sync::Notify. pending() is simpler — it creates a future that never resolves, which is all we need to simulate a permanently hanging exporter. Since we're testing the caller's timeout behavior (not the exporter's ability to be cancelled), there's no need for the test to control when the hang resolves.

PeriodicReader's timeout gap: PeriodicReader::shutdown() has a hardcoded 5-second timeout that ignores the _timeout parameter in shutdown_with_timeout() (marked with a TODO in the source). Until that's made configurable, a hanging exporter timeout test would take 5 seconds per run — too slow for CI. A comment documents this gap with an issue link.

Dependencies

This PR is based on #3380 (BlockingStrategy fix) and should be merged after it.

Test plan

• All new shutdown regression tests pass on multi_thread(worker_threads = 1)
• Timeout tests verify Err(Timeout) is returned within expected duration (~500ms)
• All 59 existing processor tests continue to pass (no regressions)
• SimpleLogProcessor's 2 ignored tests remain ignored (intentional deadlock demos)

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.3%. Comparing base (09b85b5) to head (1293251).

Files with missing lines	Patch %	Lines
opentelemetry-sdk/src/logs/batch_log_processor.rs	90.6%	6 Missing ⚠️
opentelemetry-sdk/src/metrics/periodic_reader.rs	87.7%	6 Missing ⚠️
opentelemetry-sdk/src/trace/span_processor.rs	93.5%	5 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##            main   #3382    +/-   ##
======================================
  Coverage   82.2%   82.3%            
======================================
  Files        128     128            
  Lines      24626   24825   +199     
======================================
+ Hits       20267   20453   +186     
- Misses      4359    4372    +13     

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