fix(tracing): avoid import-time tracing exporter/provider initialization

[OiPunk]

Summary

Fixes #2489 by removing import-time tracing initialization side effects.

This change makes default tracing objects lazy:

• BackendSpanExporter is now created only when first needed.
• BatchTraceProcessor is now created only when first needed.
• Global TraceProvider is initialized on first get_trace_provider() access instead of at module import.

This avoids constructing httpx.Client and thread primitives during import, which is safer for fork-based runtimes and pre-fork server startup paths.

Changes

• src/agents/tracing/processors.py
  • Replace eager globals with lazy singletons for exporter/processor.
• src/agents/tracing/__init__.py
  • Add lazy default provider initialization wrapper.
  • Remove import-time set_trace_provider(...), add_trace_processor(...), and atexit.register(...) side effects.
• tests/tracing/test_lazy_tracing_init.py
  • Add regression tests for import-time laziness and idempotent initialization behavior.

Validation

• uv run --with ruff ruff check src/agents/tracing/__init__.py src/agents/tracing/processors.py tests/tracing/test_lazy_tracing_init.py
• env -u ALL_PROXY -u all_proxy -u HTTPS_PROXY -u https_proxy -u HTTP_PROXY -u http_proxy -u NO_PROXY -u no_proxy uv run --with pytest pytest -q tests/tracing/test_lazy_tracing_init.py tests/test_trace_processor.py (22 passed)
• Changed executable lines in touched source files are covered (100%).

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Reviewed commit: af1a0f9b58

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[OiPunk]

Thanks for the CI signal. I pushed bc47daca to fix the mypy complaint in the new test helper by typing the json.loads return value via cast.

Local re-check on this follow-up commit:

• uv run --with ruff ruff check tests/tracing/test_lazy_tracing_init.py
• uv run mypy tests/tracing/test_lazy_tracing_init.py
• env -u ALL_PROXY -u all_proxy -u HTTPS_PROXY -u https_proxy -u HTTP_PROXY -u http_proxy -u NO_PROXY -u no_proxy uv run --with pytest pytest -q tests/tracing/test_lazy_tracing_init.py tests/test_trace_processor.py (22 passed)

[OiPunk]

Thanks for the review, this was a valid catch.

I pushed commit 43e8b51 to make setup.get_trace_provider() lazily initialize the default provider and processor on first use. This keeps import-time behavior side-effect free, but restores expected runtime behavior for helper entry points like agents.trace(...), which now no longer fail when tracing has not been manually initialized yet.

I also added regression coverage in tests/tracing/test_lazy_tracing_init.py for:

• trace(...) lazy initialization path
• set_trace_provider(...) global assignment path

Local checks run on this update:

• uv run --with ruff ruff check src/agents/tracing/setup.py tests/tracing/test_lazy_tracing_init.py
• uv run mypy src/agents/tracing/setup.py tests/tracing/test_lazy_tracing_init.py
• uv run --with pytest pytest -q tests/tracing/test_lazy_tracing_init.py tests/test_trace_processor.py
• uv run --with coverage --with pytest coverage run -m pytest -q tests/tracing/test_lazy_tracing_init.py tests/test_trace_processor.py
• uv run --with coverage coverage report -m src/agents/tracing/setup.py (100%)